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.