A friend of mine asked me if had a spare iSight camera to sell him. He was looking to setup iChat but was alarmed at what iSight cameras are fetching these days ($200+). That got us both on the hunt for a suitable alternative. Unfortunately, none of them match the elegance of the iSight design, but there are at least are some choices which will work. This page gives a pretty good description of all the options: Apple Mac Web Cam Video Chat Camera Site
He wound up buying the Logitech Ultra Vision SE and it worked fine in iChat right out of the box.