Try the Xbox Live Vision camera from Microsoft. It's designed for use with the Xbox 360 games console, but it has a USB connector on the end and should work with your Mac just fine. It's very cheap.
On Ebay UK there is a seller called "premier-express" where I already bought 2 webcams on separate occasions. they work in OS X and windows XP/Vista. that's for about £15. caveat: Under OS X the microphone doesn't work.
Agreed the MS xBox cam is a good bargain choice; it's a UVC compliant webcam, absolutely plug and play under OSX 10.4 Tiger and 10.5 Leopard -- As are other's listed at http://www.mac-compatible-web-cam.com/ Beware the real cheapie Kinamax cam tho, I got one of those off eBay and tho it had a sharp image it croaked on me in < 2 weeks - AND the microphone didn't work for me either. Nothing but junk, but expect boatloads of these low-cost driverless UVC cams from the Far East to flood the market soon.