Assetto Corsa is one of the best racing simulators. Good physics, great graphics, multiplayer and moddable.
Its choice of cars has everything from 50s F1 cars to road cars to hybrid Le Mans prototypes. Official AC tracks are not that many, but should keep anyone busy for a long time.
It is currently finished, its developer (Kunos) has stated that they will not release new content for AC. They are currently working on Assetto Corsa Competizione which uses a new graphics engine and features day-night cycles, rain and other improvements. Unfortunately it only features cars and a few tracks from Blancpain GT Series and its moddability is currently in doubt.
The original AC is still a very good choice for anyone wanting to play racing simulators. Also, it has a low(ish) price and is often on sale.
The multiplayer experience is good. While on random ad-hoc races you can find the odd player who crashes into everyone, there is an active community that organizes events where everyone races in a serious fashion.