Adium is an extremely cool instant messaging client for Mac OS X. It can do some pretty slick UI effects and can look very nice. It works across all the major instant messaging networks.
Migration Score
GREAT
Migration Options
Pidgin
I believe Adium actually uses the underlying Pidgin libraries to function. The feature sets for these two are not identical, but essentially you just have to put in your account information into either one and away you go.