
Recently Google updated
Google Talk to enable "federation" with other servers, the main effect of which is that you can now talk to
users on other Jabber servers using Google Talk. As a side effect, however, it's also possible to sign on to AIM, MSN
Messenger, or Yahoo! IM and talk to your buddies, all from within Google Talk.
BigBlueBall has
a tutorial on getting it working. It takes a bit of fiddling—basically you have to use another Jabber client
to set up connections to special Jabber servers that talk to other services like AIM—but once you're finished
it's easy to add new contacts on AIM, MSN, or Yahoo! to your Google Talk contact list.