The new killer app from Google, the Skype challenger is out in beta version. Now we just have to hope that it'll be cross platform as Skype. I'm on linux, so I just tested it with Gaim. Here are the instructions to make it work:
1. tools >> accounts (or Ctrl + A)
2. protocol : jabber
3. screen name : your gmail adress without gmail
4. server : gmail.com
5. click "show more options"
6. check "use TLS if available" and uncheck "Force old SSL" and "allow plaintext..."
7. port : 5222
8. connect server : talk.google.com