Trillian is an instant messanger client that handles most IM protocals. This inclues AIM, Jabber, MSN, Yahoo, and probably a good few others. Gaim does the same job, only supports more protocals and is completely free for the complete version.

Google and Gaim working together is a good thing, it bothers me that many of the feature that I enjoy in google talk are only availble if both partys are using GTalk. I understand that Google is adding extra ontop of an existing standard(Jabber) and I can't blaim anyone but google for lack of 3rd party support(Well, at least for those features). Because Gaim is open sourced, anything that is added to help with Googles extra features will be mixed into other jabber clients, and probably servers. That reminds me, I need to get a jabber server up for a few of my web addresses...