Racer is a really cool open-source racing game with a heap of cars and tracks created by different people. The realism of the game is quite impressive - the cars that have been created properly (because they come from so many sources some are better than others) behave a lot like the real thing. The physics model is very good so the reaction of the cars feels quite authentic. In fact, I would rate it better than PlayStation (and PS2) racing games I have tried (at least on race tracks and streets). In single player mode you can just race around the track by yourself and try to set a lap record, but you also can race against other cars controlled by the computer (although it doesn't seem to be a very fast driver) and against other players on a network.
The cars are fully customisable, but its not as easy to do as commercial games. All of the settings, graphics and sounds for each car are stored in standard files in a sub-folder of the game folder. By opening a control file with a text editor you can easily change hundreds of parameters such as gear ratios, drag coefficients, clutch delays, rolling, braking, toe in, and many others. Of course, if you make too many changes of this sort you aren't really driving the authentic car any longer! Plus its really easy to make a car which is difficult to control.
You can download cars and tracks for free. The cars range from hotted up Civics to more exotic machines like Ferraris and Lambourghinis, or you can go all the way and race a Formula 1 single seater. You can choose from straight drag tracks, oval circuits, Formula 1 circuits, street circuits and country roads. The rally circuits aren't particularly good so I would recommend sticking to track and street racing.
Racing in the Ferrari on the Monza track. As you can see, the car and track are both quite accurately portrayed, but the program still maintains fast frame rates (40 to 60 per second) so that the movement is smooth and natural.
Racer supports a wide range of input devices, so if you have a USB steering wheel, pedals and gear shift you can use it. Full analog steering, brakes and clutch are supported. Even using the mouse and keyboard control is good, but you really need to switch the clutch to automatic because the mouse can only do throttle, brakes and steering. Actually, this is the only racing game I have played which can be properly controlled without a specialised input device like a steering wheel, so if you don't want to invest in any extra hardware this is a good game to choose.
There are a few areas where Racer could do with a bit of improvement. The earlier versions did crash occasionally - the game I mean, not the car - that crashes a lot when I'm driving! Of course, because this is Mac OS X you can just restart the program and carry on playing and the latest versions are much more reliable. Also, because the cars and tracks are contributed by many different people with varying levels of programming skill there is a chance some of them will be buggy or of lower quality. There is a rating system when you choose downloads so it is possible to only download the items which have a higher rating. The problems I have mentioned don't happen very often, so most of the time this is a really fun game/simulation.