26. Tyrian 2000

Technically, it's been freeware for a while now, but Good Old Games' re-release of this title in December 2010 breathed new life into one of the PC's most beloved shooters. Tyrian 2000 offers loads of levels, a ridiculous amount of ship customisation, and some of the best Easter eggs around.

Tyrian 2000

If you get bored of the top-down shooting, try typing 'destruct' at the title screen to see its weapons-grade version of the classic Scorched Earth. Alternatively, unlock the ship that looks like a carrot, and banana bomb your way to victory.

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27. Wolfenstein 3D

Want to know what FPS games started out like? You can now play the classic Wolfenstein 3D from Id Software directly in your browser.

Wolfenstein 3D

Despite being released all the way back in 1992, Wolfenstein 3D is still fun for a few minutes' blasting. And you can even head straight to the final level where you battle a giant robot Hitler. It's the stuff of nightmares. And despite the kind of antics developers get up to, we're not sure they could get away with pitting you against a giant Hitler today.

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28. World of Warcraft

No, you haven't missed the front-page story suggesting Blizzard is in such desperate need it has had to make World of Warcraft totally free-to-play. But you can play the first 20 character levels without paying a penny these days.

Experts may be able to blast though these initial levels in just a few hours, but if you're yet to walk into Azeroth, it'll keep you busy for a long, long time. Watch out, though, World of Warcraft is… a mite addictive.

World of Warcraft

This freebie is also worth investigating if you haven't played WoW since the early days. While much of the content added in recent years has focused on high-end players, the whole game has evolved.

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29. Team Fortress 2

It may be an old vet in gaming terms, but nothing offers so much crazy fun as Team Fortress 2. Unlike most shooters of its age, players are still there to have a good time rather than hurl abuse at newcomers, and there's no shortage of cool toys to have fun with. Endlessly silly and amazingly fresh, it's still one of the shooter genre's kings, free-to-play or not.

Team Fortress 2

As you might guess, there are some micro-transactions involved. You can buy additional items, often used to customise your character. You can create your own. It's fun, and gets you even more involved in TF2. Those cheeky devils at Valve know what they're doing.

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30. Reprisal

Populous returns in Reprisal – a gorgeous pixel-art reinvention. As a God, use your powers to build a civilisation and crush all who oppose you – but don't think magic powers will make things too easy. Best of all, it's playable right from your web browser.

Reprisal

The game's maker said Reprisal was made as an homage to the early God games. There's now also a larger, paid version called Reprisal Universe – but there's more than just a taster on offer in the browser version.

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31. Realm of the Mad God

Online RPGs have never been so streamlined, or so insane. Join groups of up to 85 players to fight through an insanely lethal world that borrows as much inspiration from bullet hell shooters as hack and slash action games.

Realm of the Mad God

When you die, you die for good... but Realm of the Mad God is so fast that rolling a new character and jumping back in from the start is no real hardship.

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32. Alien Swarm

Did you know that Valve, maker of Half-Life and Portal, made a multiplayer twin stick shooter? Well it did.

Alien Swarm

Alien Swarm is a co-op experience that sees you face-off against hordes of aliens, and it really has more in common with Left 4 Dead than the RPGs that are commonly called free-to-play. The look is quite different, though. You see the action from high above, not that it's enough to stop the white knuckle effect when the blasting really heats up.

This is a gem many people have missed out on, so look it up on Steam now.

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33. DC Universe Online

DC Universe Online lets you create a hero and leap into action in the streets of Gotham and Metropolis, either backing up Batman or signing up with your favourite super-villains.

DC Universe Online

The outside street action is pretty bland, but the dungeon design includes just about everyone from the DC Universe to fight or team up with, and is just as much fun whether you want to form your own Justice League or live the life of a super-powered lone wolf. (But not Wolverine.)

It's one for superhero fans only, perhaps, but who wouldn't want to try out life as a flying super villain for a few hours?

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34. Fallen London

Descend if you dare into a vision of Victorian London abducted by a swarm of bats and moved to the edge of Hell. Fallen London is the digital equivalent of a classic 'choose your own adventure' book.

Fallen London

Slightly simplistic mechanics don't spoil a gorgeously written world of demons and social intrigue, and while there are some social elements, you don't need to annoy friends to make the most of your new life in this surreal underworld.

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35. Auto Club Revolution

There are plenty of free-to-play driving games out there, but one of the few not tied to a single car-maker is Auto Club Revolution. Instead, there are two – it was made in association with BMW and Renault.

Auto Club Revolution

The game lets you drive some of the world's best cars for free – especially if you play through the BMW Experience, which gives you the Series M Coupe as your starter vehicle. Auto Club Revolution features racing, driving for pleasure, and a huge community waiting to welcome you onto assorted real-world courses.

If you want to get your hands dirty, you can also customise each car with actual parts, and give them a full makeover with decals and other neat touches.

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36. Spelunky

You can now get Spelunky on all sorts of platforms – it's pretty high-profile for an indie title. But it began its life PC-only, and it's this original 'non HD' Classic version you can still get for free today.

Spelunky is about anger, hate and, most of all, death. It looks like a simple enough platform game – an Indiana Jones pastiche set in a cavern full of tricks and traps – and it is.

Spelunky

There's nothing complicated about it. Every enemy is avoidable. Every trap can be dealt with.

The catch is that every time you play, the entire game is randomised. In one game you'll stumble through screen after screen of spiked horrors and swarming monsters; in the next, the software will bend over backwards to give you gold and help you on your way.

The trick is learning the ropes, figuring out how to get past every obstacle, and then doing so perfectly as and when the game throws things at you. You will die. You will die a lot. But the important thing is that in death, you learn.

You discover ways of stealing from the shopkeepers who inhabit the levels, or find out that the damsels you can rescue for a health boost can just as easily be taken to the nearest sacrificial altar, or thrown around to trigger traps before you go down yourself.

You learn how each randomised world ticks and which equipment will give you a fighting chance. And then you'll die some more. And scream. And restart. Again.

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37. Super House of Dead Ninjas

This is one of the most enjoyable Flash games in recent memory, which is also available as an expanded commercial-but-cheap release on Steam if you fancy more toys and full-screen action.

Super House of Dead Ninjas

The free version of Super House of Dead Ninjas feels like a complete game in its own right though, as you guide the Crimson Ninja from the top of a demon-infested tower to the horror waiting at ground floor.

Randomly generated adventures keep things fresh, with the speed of the action more a challenge than any individual enemy. You can handle any situation in front of you – you just don't get to stop to catch your breath. Ever. Until you die, of course.

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38. Command & Conquer: Tiberian Sun

Command & Conquer: Tiberian Sun got a bad rap at its original release back in 1999. It was too slow and buggy, people said, but many of the issues were patched out. If you remember turning your nose up, it's time to take another look.

Tiberian Sun

EA made the game freeware to celebrate the release of Command & Conquer 4 back in 2010. It didn't work too well – C&C 4 hardly resurrected the brand.

In standard EA fashion, Tiberian Sun is no longer widely available from Origin, the EA Steam equivalent, but you can still find the freeware installer package for the game and its expansion online.

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39. Quake Live

This is how far the web has come: one of the best deathmatch games ever created is now available to play from within the confines of your browser.

Well, technically, no; it's not actually in it – Quake Live uses a plug-in and then goes full-screen when you play – but the spirit is still there.

Quake Live

Any modern computer is now able to handle Quake 3's modest demands, and the game's blisteringly fast action makes it quite unlike any modern shooter. Forget realism. Forget objectives. Sometimes, all you need is a rocket launcher, a perfectly timed shot and the lamentation of the noobs as time permits.

Beware, though: if you haven't played id Software's classic shooter for a while, the frenetic pace of the online action might be terrifying.

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40. Beneath a Steel Sky

Beneath a Steel Sky is a classic adventure from British developer Revolution, the maker of the Broken Sword games.

It's a sci-fi adventure with more than a hint of Blade Runner flavour. Like all the best point 'n' click titles, though, there's also more than just a little humour in the script.

Beneath a Steel Sky

A Remastered version is available for iPad and iPhone, but it's the original you'll find on Mac and PC. While the game runs through the ScummVM system, you can grab it on GoG to avoid any fiddling about.

If you're still thirsty, check out Lure of the Temptress, another Revolution adventure.

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41. Slender

The question of whether games are art or not is a dull debate that has raged on for years. But making you terrified from one minute to the next is an art in itself. An art Slender has down.

Although it's just a simple 3D exploration jaunt where you look for eight pages seemingly scribbled by the Slender Man's victims, this game is terrifying. Our monster in this little slice of horror is a tall faceless man who stalks you, hunts you.

Slender

Set in a dark forest with nothing but a flashlight to keep you company, if this doesn't give you chills, nothing else on this list will. Once you've completed the Eight Pages, you can also check out the slightly beefier horror-adventure Slender: The Arrival. It's not free, but is a good way to test your nerve.

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42. Kingdom Rush

Nuggets of tower defence gaming are perfect if you want the sort of light strategy that gives you a feeling of power without sucking away your whole day to provide it. And Kingdom Rush is one of the best tower defence experiences you can get.

Kingdom Rush

As well as being available on mobile phones and tablets, you can play it directly in your browser. It's a true smash, offering great balance and more charm than a half-dozen other free TD titles combined. There's also a sequel available, Kingdom Rush Frontiers, when you've picked the bones of the original clean. Which you probably will do if you fire up Kingdom Rush. Trust us.

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43. Digital: A Love Story

Remember the excitement of logging into your first BBS? What if you'd found something more than just files and chatter and naked pictures of assorted Star Trek actresses?

To explain Digital: A Love Story would be giving away too much, so let's just say that it's a great nostalgia trip with a bit of future-gazing thrown in for free. Played out entirely on 1988-style bulletin boards, it starts when you respond to an email from a lonely sounding girl called Emilia.

Digital a Love Story

The relationship plays out as a hacker's romance as you jump between BBS systems to uncover a conspiracy, mostly interacting by firing off emails to the characters. You never get to see what you've said, only the responses, which adds an unusual but effective disconnect to the conversations.

It's not a long game – only an hour or so of action at most – but it's a testament to the writing that you quickly get sucked into what is basically just typing out a lot of phone numbers. The authentic sounding music and sound effects help: the sweet siren song of a modem connecting still sends a chill down the spine.

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44. Neptune's Pride 2

Where some free-to-play games want to consume hours of your life every day, Neptune's Pride 2 only wants a handful of minutes.

It's an intergalactic version of Risk you play with real people, over a period of weeks or months. You can forge alliances and work together, but every player has to know there can only be one winner… the one who 'owns' more than 50% of the galaxy.

Neptune's Pride 2

Every day you earn more money and make your strategic decisions. It's like chess, but with star systems instead of pawns. If you want to deep-dive into a game with some friends, Neptune's Pride 2 is great. But be warned: this stuff can ruin friendships.

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45. Dwarf Fortress

If you find games like SimCity or Civilisation a little too simple, Dwarf Fortress is the game for you. Technically, its full name is Slaves to Armok: God of Blood: Chapter II: Dwarf Fortress, but absolutely nobody calls it that.

It procedurally generates a whole work, complete with its own history, in very (very) basic visuals. But to get hung up on the graphics is to miss the point of Dwarf Fortress.

Dwarf Fortress

You can build your own fortress, or go out adventuring. It is not easy or at all forgiving, though. When adventuring, it's roguelike in style, meaning that when you're dead, you're dead. Casual this is not. And as the game looks like it has been drawn with a typewriter, you will need to use your imagination a bit.

However, Dwarf Fortress is quite unlike anything else on this list.

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46. Sarien.net

Remember the Sierra adventures of old? Sarien.net revives them, and makes them multiplayer – at least, partly. Technically, you still play on your own, but you can see other players wandering around the world as you do. That said, there were only a few players online when we last tried the site.

Sarien.net

Sarien has access to King's Quest I-III, Space Quest I-II, Police Quest, and a lesser-known game, The Black Cauldron. These are titles from the early days of games, so be aware that the visuals are not going to wow you. For a better-looking adventure, check out the remakes of King's Quest, available online.

We've hesitated to mention this one before due to legal questions over it, but now it's been officially approved by Activision, there's nothing stopping you from jumping right in.

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47. Vindictus

The sequel to a Korean MMO virtually no one we know has even heard of, Vindictus doesn't have the clout of something like Dota 2, but it offers something quite different too. The visual style is distinctly Asian, giving it an unusual feel in a world of fairly West-friendly free-to-play role-players.

Vindictus

It is an online RPG, but one focused on hack and slash action over questing and levels. It looks great thanks to the Source engine, and the combat is enough to get you past the inherently grindy nature of much of the progression curve. Arachnophobes beware though – the tutorial has one of the biggest spiders you've ever seen, and yes, you do have to get right up into its face to fight it.

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48. Desktop Dungeons

Ah, the quest you can complete without ruining your appetite for monster slaying. Desktop Dungeons is as simple as heroics get – really, the entire game is about fighting your way up the local monsters' organisation chart without picking a fight with something capable of crushing you back.

Desktop Dungeons

It's Rogue in style – meaning when you die it's really game over – but without the usual complexity, and it's still very moreish. This is the perfect way of killing a boring lunchtime.

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49. Cave Story

This is a classic Japanese freeware game with a lot of shooting, even more jumping, and a five year development time that still barely explains where all the great ideas came from.

Cave Story

Cave Story is a little fiddly to get running, but an absolute must-play that's influenced a great many other indie developers since it came out.

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50. FlightGear

With the new version of Microsoft Flight Simulator not arriving until 2015, if you want to indulge the aviation nerd within you, FlightGear is currently one of your best bets.

FlightGear

FlightGear is free, with extensive terrain mapping, lots of aircraft, support for multiple desktops, and more. You can even download the source code if you want to get deep into its guts.

It's not the prettiest game, mind, and if you're after action you're much better off with something like War Thunder. This is a flight simulator, not a war game.