http://www.microsoft.com/mac/default.aspx?pid=msnmessenger
MSN 5.0 (for Mac) looks rather nice. The features page says it can be used across different networks (!)—anyone got this to work/know what it’s about? Update: you need Live Communications Server, so basically corporates only. I didn’t realise Microsoft was officially supporting other IM networks. (Or that other networks would let them.) Curious.