Rigs of Rods is an awesome open source, FREE, car simulator. Well actually, it also simulates all kinds of trucks, boats, trains, planes, forklifts, and, um, oh yeah, a crate. So basically anything you can think of. These vehicles are mainly created by the community (you can download them from the Repository as "mods"), so that the developers can focus on more of the game itself.
Again, this is a simulator, not a game, so there isn't a story. But there is a Multiplayer mode, and many maps created by the community are great for roleplaying, such as MitCity. There are also quite a few RoR Multiplayer servers that have this map. But still, although there is no plot (you're just driving), the developers somehow made it really fun to play.
Okay, okay. So the graphics obviously are not as good as NFS for other car games, but they are quite good enough. You can find a few maps by the community that look very nice, but due to the already somewhat high system requirements of RoR, they would be pretty laggy. A computer that can handle every vehicle and terrain without lag needs to have at least 8 GB of RAM and a quad-core processor.
Overall, this simulator is pretty cool. In fact, I hardly played any other driving game after finding out about this masterpiece. I encourage you to go check it out here!