While more people seem to use Trillian, Gaim was a nice open-source alternative IM aggregator, and had the advantage of being able to incorporate Google Talk alongside AIM, MSN and Yahoo chat. Seems that after a long battle with AOL, Gaim has had to change its name to Pidgin. You can read about it at http://www.pidgin.im/ (or at http://gaim.sourceforge.net/). On the plus side, there should be a new release of the software later this week.