GAIM is such a good IM client - I've been using it for a few months and with the release of v1.02 I think it outstrips MSN Messenger in all areas. Aside from support for ALL of the messenger and chat protocols (something the boys in Redmond show no signs of embracing!) it has a cleaner interface and a very extensible philosophy - it supports pluggins (much like Firefox) and also tab'ed conversations (much like Firefox). It keeps a text file record of conversations and with the latest release has your buddy picture (something my children thought was important!).
The only slightly insulting thing is how it describes people who haven't touched the keyboard for a while!