Play with your Xbox 360 controller on Mac OS X
An enterprising Mac gamer, Colin D. Munro, created a USB driver which adds support to Mac OS X for the official Microsoft Xbox 360 wired controller. Along with all the buttons, both analog sticks, the analog triggers, and the D-pad, it also implements Force Feedback support using the rumble motors as well.
Third party controllers should also be supported, but these must be added manually to the driver's list of supported devices. Currently, the official controller and the DOA4 stick controller work out-of-the-box. Note that wireless controllers won't function -- they communicate with the XBox 360 using a proprietary RF protocol, so the Bluetooth module built into your Mac won't talk to them.
The driver is still a bit buggy and some features like the ability to alter HID descriptor (both to remap buttons and change device type) and support for all 360 controllers (without adding each one to the plist file manually) are missing. However, if you don't use a Windows dual-boot system to play games Colin's driver is definitely worth a look.
The driver may be found here -- make sure to check which revision is suitable for you!