I have created a USB driver which allows you to use wired XBox 360 Controllers via USB, and wireless XBox 360 Controllers via the Microsoft Wireless Gaming Receiver for Windows, on your OSX machine, including support for the Apple Force Feedback library. The driver is licenced under the GPL.
I've released a version which will hopefully install and work fine on 32-bit Snow Leopard. It also contains 64-bit binaries, however I've been unable to test them because Apple have disabled my MacBook from booting into 64-bit mode. I have however been informed that 64-bit and 32-bit builds are both working.
I have got the Microsoft ChatPad working with my wired controller. The latest release of the driver includes support, and I'll be updating the USB information section of this website shortly. I've not yet checked the wireless receiver for compatibility.
Sadly, my PowerMac has died, which as my primary development machine has slowed progress.
I have added a version of the driver without support for the Guitar Hero controller, to allow the Guitar Hero for Mac game to work (it attempts to access hardware directly, which doesn't work if a real driver has claimed the device).
If you find the driver does not work for you, please attempt and find out as much as you can about the device, preferably using the Apple "USB Prober" application provided with the developer tools, but the output of System Profiler for the device may be enough. E-mail it back and I'll try and work with you to get it working.
Force feedback-enabled games
Games I've currently tested for force feedback support (only games that support basic rumble will probably function currently, as I've only implemented triangle, square and sine wave-type effects. I also lack any other force feedback device for comparison :) ):
Jammin' Racer - seems to work fine
Here's other similar drivers that are different versions or releases for different operating systems: