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.
Not sure, GTalk the Windows app at least is not compatible and the browser plugin seems not to be either, because Google uses a pre-standard version of Jingle that is not supported by Gajim. They claimed to update the client.. some years ago, nothing happened.
I think that Jitsi (formerly named SIP Communicator) supports audio and video chat on Windows and OS X as well.
https://jitsi.org/