Trillian. It's priceless. (Literally - it's free.)
No ads, and can run 5 different messaging programs at once. Slim, as in it doesn't eat up resources like ICQ does.
www.trillian.cc
Great program. About the only thing I can't get it to do is transfer files, and that's simply because I don't want to bother with going around my firewall here at home. Folks can always upload the files or email 'em to me.