I have a Pico iMage camera that works well on Macs, Linux, and Windows. I use it with Skype (Linux) all the time and people seem to think it looks just fine. Here is a quote from a review by MacNN:
The iMage, produced by Pico Instruments, boasts a 1/7-Inch color CCD sensor that presents 640 x 480 (VGA) and 320 x 240 (QVGA) images at 15 frames per second. It works well in iChat. ... it requires an USB 2.0 high speed port... It has built-in white balancing and auto electronic iris control, which accounts for the high quality color.