Jabber

[Will]

GAIM is cool too. It's just that Trillian is oh-so-much more purdy. /me pets the Trillian skins... GAIM (at least when I tried it out) had an ugly Java-type look, and with the setup I had, I wanted to utilize the graphics processor, not let it stand idle

And I like a bunch of the plugins (News, Weather, Clipboard Ext, etc).

[Aiken]

In the discussion of multiprotocol IMs, I think you shouldn't forget about Miranda. Best thing in this IM is 100s of plugins which can expand the functionality up to eternity (well, almost). However it's not a ready-to-use solution, but it suppose user to be a geek.
Oh, and Miranda is GPLed plus it has a jubber plugin too.