iChat

iChat has many of the same features as Skype, excellent voice quality and ready integration with iSight video cameras. It is also less of a bandwidth hog than Skype. The current version, iChat AV for Mac OS X 10.4, is the next best thing to being at a conference table with your co-workers, enabling razor-sharp videoconferencing with three others or audioconferencing with nine. In October, a new version of iChat will accompany the release of OS X 10.5 and add more bells and whistles; see this demo to find out what it can do.
Pro: Extremely crisp audio and video built into every Mac.
Con: You need a Mac.