There are few decent driving games on the Mac and even fewer that are free. Racer is definitely a decent attempt at creating the latter, although I came across more than a few problems with it.
Racer is a cross-platform car simulation project which is high on realism, employing accurate car movements and controls to maximum effect. The fact that it's open source means that what you download are only the basics - to add more cars and tracks you'll have to go to the contents page where you will find links to several sites providing more cars and tracks.
When you start Racer, it's best to go to 'Options' and then 'Controls' to configure how you'd like to control your car. Click on the aspect you want to change the control for (such as accelerate and brake) and then press the button you'd like to use for it. When you start a race, you're able to configure how many cars you'd like to race against, which track, which car and how many tracks. However, you have to download these options from the contents page and upload them to Racer. It would have been nice if the developer had included at least a few examples to give a better taste of what the game can do. And besides, when I tried to select 16 cars to race against, the game simply crashed.
Because the controls are designed to be as realistic as possible, handling your car is quite tough. To get started, get your revs up and then shift into first gear. The problem is that the gear indicator isn't very clear, so as you're racing it's hard to know what gear you're in. However, the sounds and controls give a feeling of realism and the graphics aren't bad at all for a free racing application.
The good thing about Racer is that it is open source and the sky is the limit for those wanting to add to it. The bad is that you have to visit different sites to add bits onto it, the controls take some getting used to, and it is still prone to crashes.