On linux Empathy, Gajim, Jitsi, and Pidgin all support audio and video chat. I believe they're all interoperable with each other and with the video chat in gmail.
I think that Jitsi (formerly named SIP Communicator) supports audio and video chat on Windows and OS X as well. I don't think that any of these projects support video conference calls yet, but it's on the "near future" section of Jitsi's roadmap.
