Original code playing nice with Windows XP and beyond
The Doomsday Engine
The original Doom's a curious game. At the time it was released, it was scary and pacey, but at some point in the last 15 years, as graphics have improved, it has became a high-speed, cartoon action game that feels endlessly fresh.
There's good reason why its community aren't prepared to let it fade away. Doomsday is huge, ambitious project that a) ensures it runs just dandy on modern operating systems and with modern controls b) optionally sparkles up the graphics into hardware accelerated, sharp edged 3D, dynamic lighting and, optionally, replacement character models and c) makes online multiplayer easy-peasy.
That said, the current king of deathmatch Doom is Skulltag. Doomsday has the added benefit of supporting Heretic, Hexen and Doom 2.
A virtual machine that's probably a little worried about itself now that Lucasarts has gradually begun remaking and re-releasing its classic early 90s adventure games, but until the likes of Sam & Max, Day of The Tentacle and the later Monkey Islands get an official spit 'n' polish, this is hands-down the best way to revisit them.
It's the beloved comedypuzzlers as they originally were, but with zero-compatibility problems and a raft of upscaling options for our gigantic contemporary monitors. ScummVM has lately expanded to support non-Lucasarts games of the era too – the likes of Beneath A Steel Sky, Gobliiins and Simon The Sorcerer are very much at home in it.
There are also ports for PDA, iPhone, DS et al, get it here.
Exult: Ultima VII
1992's seventh in Richard Garriot's high-fantasy Ultima series is still considered one of the role playing genre's highest watermarks, so its fans are resolute that it not be lost to history.
Exult doesn't do anything flashy, it simply gets this olden DOS game running as well as possible on today's PCs. It also fixes up a few bugs and back-ports new features from Ultima VII Part 2 to the first part, as well as introducing support for mods. You'll need the original Ultima VII date files to use it, but it's well worth an eBay-scour to discover quite how phenomenal a game it was. Get it here.
This is essentially a catch-all generalist project for any and every old DOS game that grumbles at modern Windows operating systems.
Unlike the new engines above, it doesn't do anything to the games bar run them well (most of 'em, anyway. it has a few blind spots, which are gradually being fixed), but that's enough to keep PC gaming's vast archive alive and playable to this day.
It does require a spot of command line action, but worry not - graphical frontends are available if you're an idiothole. Anyone with a genuine interest in PC gaming absolutely has to have this installed.
Slightly underwhelming sequel to beloved first-person-shooter series? No problem! Modders have addressed almost every annoyance of Doom 3, from the ridiculous inability to use a light at the same time as a weapon, to sorting out textures that have aged pretty terribly to adding the co-op mode that was so mysteriously absent from here and here.
Fake factory's Cinematic Mod performs wonders on the old Source engine. It manages to improve the lighting, bump up the visual detail and, a little controversially, replaces the Alyx character model with a more a realistic version, based on a choice of real-life models.
Rather tragically though, one of these does appear to have a see-through top. Simply leave that option off and enjoy a far more believable City 17.
Oh how we've waited for this one. Just a few weeks ago, our prayers came true: a mod that allows the definitive hack 'n' slash RPG to be played at resolutions above blocky, blurry 800 x 600. It looks crisp and lovely at high resolutions, but if the expanded world (and the limited light your character's sight radius casts over it) feels a bit too at odds with Diablo II's nature for you, what you will enjoy is being able to play the game in a window at last.
The bad news is that Blizzard's Battle.net system currently sees it as tampering with the game code and forbids online play while it's running. For single player fun though, it's perfect. Grab it from here.