This morning Apple, INC made several announcements, including availability of a beta FaceTime application for OSX 10.6 and later. As I mentioned, FaceTime is a Snow Leopard only application and requires an account with Apple to use (i'm using my MobileMe account, but I believe any iTunes Store account would work as well).
The experience is pretty good, I was able to quickly do a facetime chat with my brother, both from the desktop and my iPhone4 (he was on the desktop beta for both. The quality from the phone was much better, but that's because I'm using a 3rd party webcam on my MacPro which is much lower quality than the current line of built-in cameras on the Macbook Pro's, iMacs, and the Monitors.
Like the iPhone4, this is most likely a streaming h.264 video stream, though i'm unsure the specs being used for the encode, i'll root around and see if i can figure that out this week and do an update.