Skype, today, released two treats for us Mac users: Skype Beta 1.5 and Skype Video preview. Both are basically the same program with the exception that that the Video preview supports video chatting, though it is in it very early stages so it will crash (the Beta is more stable).
New in Skype Beta 1.5 is a totally new (and Macified) UI, detachable call window, the ability to import contacts from Address Book and Entourage, and a bunch more. Check out the changelog for the full dope.
Skype is free.