Version to use: 1.2.0 (though they update often, I may be out of date here...)
GAIM provides a multi-client chat interface. That means that if you have accounts on any of the chat platforms, you can use GAIM to access that account. It isn't magic, you can't talk to people on MSN if you have an AIM account, but if you have both, you can run one interface and still get to all those people. It also provides fun things like RSA encryption via the Gaim-encryption plug in. There's lots more options investigate them.
Recently, I found out I'm not supposed to use outside IM clients. Instead, we use POD which is, um, bad.
Also, Google has released < a href="http://www.google.com/talk/">Google Talk which has the benefit of a decent UI, integrating with my GMail contacts, hopefully opening up IM protocols to standards, and providing VOIP. Not half bad. If you need an invite, let me know and I can help you out.