Founded in 1992, ISI are a well respected software developer specializing in the fields of computer game development, "man-in-the-loop" simulator architectures, computer image generation, and entertainment systems integration.
Products include Sportscar GT, EA Sports Formula One titles and rFactor.
Download, try and purchase within the game (no other download necessary). The rFactor trial includes cars and tracks for evaluation, and is unlocked after purchase. A full run-through of how it works can be found here.