It's a tossup for me between Proteus and Adium on the mac. Neither is near as good as Trillian. Both have different strenghts and weaknesses, but neither is as mature as Trillian. Given time, I'll bet they become pretty nice to use. It is nice to be able to have all, or most, of your IM clients all rolled into one, regardless which platform you are on. Gone are the days of AIM, ICQ, MSN, and Yahoo all being open in seperate mind and memory sucking programs.