Pidgin (earlier Gaim) is a multiple protocol Instant Messenger client which can be used to chat using MSN, Yahoo, AIM and many other protocols. But most people do not know that you can use Pidgin to chat with Google Talk users.
Pidgin supports a protocol called XMPP (Extensible Messaging and Presence Protocol). XMPP is the core protocol of the Jabber protocol and GTalk is also based on the Jabber protocol. So it is quite easy to make Pidgin to work with GTalk.
microblog-purple is a plugin for Pidgin that works both on Windows and Linux, that supports Twitter, Identica, and Laconica-base server through the conversation windows.