Tillian has made a name for themselves as an integrated IM client. With so many different IM services out there it is nice to have everything under one roof. Trillian supports AIM, Gtalk, IRC, Mobile Me, Facebook, MySpaceIM, Yahoo all from a single app. Other goodies that Trillian has thrown in are emoticons, photos, ping a friend to grab their attention, tabbed chats, and landscape mode.
The release is still under beta but I didn't notice any glaring bugs after adding several accounts. Grab it while you can for fee, rumor has it that it will be a paid app when the final release comes out.