A Company in France has released a copycat iSight webcam. Makes the same claims as far as specs (640x480 resolution / 30 frames per sec) as Apple's iSight, but doesn't include a built in Mic and runs on USB 2.0 instead of firewire.
The only way you could get it working on the Mac though is to use iChatUSBCam
My experience with "other webcams" is that they perform very poorly ... for some reason no cam is as fluid (on a Mac) as the iSight. It is truly real time video conferencing.