Talk talk.
So Google Talk is live. It appears to be a Jabber service at talk.google.com for GMail users (try it! your Jabber ID is you@gmail.com) with some VoIP extensions for you if you use the Google Brandâ„¢ Jabber client (Windows only).
[If you want to talk to me, my JID (and therefore my GMail address) is my first name (short version), dot, last name, at, gmail dot com.]
Also be sure to check out their Developers FAQ, which offers some slightly deeper details on the app:
4. Do you plan to support other real-time communication protocols?
Google Talk supports XMPP with the beta release. We plan to support SIP in a future release. Additionally, we will evaluate other protocols as appropriate, to continue to deliver on our commitment to open communications.
5. What protocols are used for voice calls?
Google Talk supports a custom XMPP-based signaling protocol and peer-to-peer communication mechanism. We will fully document this protocol. In the near future, we plan to support SIP signaling.