For those of us not lucky enough to get a built-in iSight with our PowerBooks like those MacBook and MacBook Pro owners, the only other practical choice is the relatively expensive iSight. But if you don't want to fork over $149, here's the iMage USB webcam. It gives you 640x480 video (like the iSight), and works with iChat AV, Yahoo, and Skype Beta.
It's USB compatible, and works without any need for drivers. The included software gives you added functionality, but by default it works like the iSight. Available for $59.95, which is almost a third of the price of an iSight.
iMage USB webcam [ecamm]