Unlike many USB web-cams made for Microsoft Windows, few are truly compatible with the Apple Macintosh platform. Make sure to check for the Mac logo on the box when purchasing a web cam, or look for info stating the camera is UVC - USB Video Class compliant.
On modern versions of OSX 10.4 and 10.5, UVC web camera drivers are built-in to Apple's Operating System. Hence, any UVC compatible web cam is truly driver-free, plug and play on the Mac platform.
As an alternative: Apple has long supported FireWire - 1394b webcams and DV camcorders. So some people do their video chatting with a FireWire camcorder instead.