For the past six months or so my preferred IM client has been Psi (that's pronounced "psee" if you honor the ancient Greeks). For a long time I resisted using Psi because I didn't like its ICQ-ish appearance in the old days, but in its more recent versions (such as the just-released 0.11) Psi has several significant virtues as far as I'm concerned:
So here's to Psi 0.11 -- I'm looking forward to 0.12 (tabbed groupchats!) already. :)