In Windows, I use Trillian for AIM, ICQ, IRC and MSN, and Yahoo! Messenger because Trillian's Yahoo support always sucked, perhaps not anymore.
In Linux I use Gaim for AIM, ICQ and MSN, and BitchX for IRC.

Edit: Fuck, that was supposed to be a response to the IM thread..Lil, can you move it? Pretty please?

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I want to know everything
I want to be everywhere
I want to fuck everyone in the world
I want to do something that matters