I know it was already declared unanimous, but I would like to add my vote for BeejiveIM. I would also like to add that the folks at Trillian have an incredibly lengthy development cycle. TrillianAstra was in beta forever...
Does anybody use AIM or anything like that any more? I'd love to chat like that with friends, but it seems that Facebook is a lot more convenient for most of them.
Def BeeJive, great app, use on my Blackberry and iPhone. Facebook is a lot more convenient for most and BeeJive allows you to connect to it easily as well as all your other IM clouds.