Digbsy is a must have application, IMHO

Digsby is yet-another-multi-im tool. I love it, it has support for facebook chat, twitter, gmail, msn, g-talk and so on. Altogether in one client which looks nice.. Even supports myspace, but that's so 1990's these days.

File transfer isn't working for me tho. I like to dock it on the left side with auto hide enabled. You can combine multiple contacts (ie same person on msn, facebook etc) into a single contact in the list which really cleans things up.

It works on Linux, OS X and Windows as well and you get growl-ish bubble type notifications on windows from it :) (Updated not quite yet for no windows)

Check out the features through the screen shots

I have recommended it to many friends and they all loved it.