rFactor was first released in 2005 for Windows, and did away with the expensive real-world F1 licences in favour of pitching itself directly at the simulation nerds. Developed by Image Space Incorporated, the game is perhaps not the most accessible to newbies, but offers lots of flexibility.
The game was particularly praised for its tire modelling, which at the time was viewed as the best out there, as well as its approach to aerodynamics.
There are thousands of mods available for the game, so you can configure things exactly how you want them, and the software has even been used by industry to build simulations. Of the many unofficial mods, perhaps unsurprisingly some of the most popular port over tracks and real-world vehicles from other games.
Some updates to the official game also started out as mods, such as recreations of a number of famous circuits including home of British racing, Silverstone, and the notorious Nürburgring in Germany.
A sequel, rFactor 2, was released in 2009 and introduced even more advanced physics, as well as weather effects and better shadows. The game is so realistic that it's even used to teach real-life trainee racing drivers; apparently drivers who are part of the Gran Turismo Academy programme, which is jointly run by Sony and Nissan, use rF2 instead of the game that gave the programme its name.
One mod developer, MAK-Corp, has even claimed that its clients will ask it to build new components as mods for rFactor 2 so they can be tested in-game, long before they're built and tested in real-life cars.
Perhaps the only area where rFactor 2 doesn't fully succeed is graphics. James Dover says that, in his opinion, "rFactor 2 has an incredibly diverse physics engine and it's very customisable, but it doesn't look as visually stunning as Assetto Corsa or Project Cars".