Google Talk...
xmpp google gmail jabber
It looks like everyones talking about Google's forth coming XMPP/Jabber server at the moment, Brian Del Vecchio has a page showing connection details for Gaim.
The guts of it are that your "JID" (Jabber User Id) is the same as your Gmail email address, but the Jabber server is talk.google.com. Not all clients appear to let you use a different server than whats in the JID (Gaim and Gossip both do) so you may need to hunt around.
Currently I've been able to talk to other Gmail users ONLY after they've been added to my roster (good for anti spam) and as yet no form of server-to-server comms - this may be due to Google not providing s2s, but may also be related to using a different server hostname than whats mentioned in the JID (I'm unfamiliar with now this actually works, so that may be a non-issue).
This'll be an interesting thing to watch...
Update: Google themselves now have an official page showing gaim settings.
Looks like Google is going to do s2s, but they're going to be selective about who they connect with:
http://www.google.com/talk/developer.html#service_2 "We look forward to federating with any service provider who shares our belief in enabling user choice and open communications. We do believe, however, that it is important to balance openness with ensuring that we maintain a safe and reliable service that protects user privacy and blocks spam and other abuses. We are using the federation opportunity with EarthLink and Sipphone to develop a set of guidelines by which all members of the federated network can work together to ensure that we protect our users while maximizing the reach of the network."