Google talk
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.