I haven't recommended anything in a while, so here goes.
Trillian is, to all intents and purposes, MSN, AIM, ICQ, Yahoo! IM and IRC rolled in to one program. Since everyone uses MSN, this may be of interest to you. It offers two killer features that, in my opinion, make it a worthwhile download even for single-service users.
- It remembers your last status, so you can log in whilst appearing offline.
- It logs all of your conversations.
So, if, like me, you are an antisocial nostalgist, you should download Trillian. Now.