Google has launched Google Talk. It's a Jabber service with some VoIP on top. The latter can only be used with the Google Talk client, but the rest should be completely usable with a number of clients, probably any Jabber client.

I tried with Fire—which is not in the above list—and it just worked. I connected to talk.google.com port 5223 (the default) and used my GMail account name and password. I also checked the “SSL” and “Verify server certificate” checkboxes, so I don't know if it works without. I also don't know why you'd use it without SSL.