A year ago, I bought a logitech's orbit and I didn't like it at all. It simply didn't work properly with my PowerBook. I sold it and I bought an iSight.
I hope there's something out there cheaper but I wouldn't dare to try it.
I would save until I have enough for an iSight.
iSight is the only webcam worth having. There are a few others connected by USB and FireWire but none come close to giving the great quality that iSight gives.