I use GAIM for all my instant messaging.  I used to use Trillian, but I switched.  I use MSN, Yahoo, AOL, and GoogleTalk (Jabber).  I dropped ICQ some time ago because no one I knew still used it.

Recently, some MSN server changes caused GAIM to crash when logging in.  The GAIM team has fixed this bug and has released a beta with the bug-fix.  I've installed it with no problems, and I'm on universal IM once again.