Refurb iSight Webcams
Old Macs w/FW 400 Port
Modern UVC - USB 2.0 speed video class web cameras are what you need for real Mac compatibility, and on OSX you need to be running OSX 10.4.3 at the ABSOLUTE MINIMUM for UVC webcam support. It's important to note that UVC also implicitly requires a USB 2.0 HIGH SPEED port or you won't see any image at all when connected. So while you may have a Mac that is even capable or running OSX 10.4.x, you may not have a fast USB 2.0 port and be stuck with unworkable USB 1.1 ports instead. In general (there were some exceptions) - When G4 processors hit 1GHz+ is when Apple rolled out USB 2.0 support. If you have a G3 processor or a G4 with CPU speed is 800Hz or less, UVC webcams for Mac just won't work. Period. Single or Dual CPU G5's are a much better match that's still somewhat workable for video capture and conferencing.
1.3MP VGA Webcam
For Older G4/G5 Macs
Lower resolution, 640x480 SD - Standard Definition cams are less demanding of CPU and internet bandwidth, so for older USB 2.0 Macs, the above Creative Lab's VGA webcam is an excellent choice. Frankly, if you're still using Mac's this old to do your day to day stuff on, you're living in the past as a hobbyist or collector. You probably have far bigger issues than finding a Mac compatible web camera. Even if you do, the OSX video chat, capture and conferencing apps that run on OSX 10.3 or 10.4 are so out of date you can't even connect to the chat services. Web based video chat using ancient versions of Flash Player inside of your web browser or other means probably won't work either.
For the rest of us living in the ruthless advances of Technology in the Zen of 'Be here, Now' -- Modern, high-definition 720p and 1080p HD webcam for Mac options are your best best on Macintosh systems with dual or quad core Intel processors.