Recently Google enabled "federation" with other servers in googletalk, the main effect is you can now talk to users on other Jabber servers. As a side effect, it's also possible to sign into 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.