Pidgin is a good one, try it, it is nice ;)
One of Pidgins strongest selling points is its support for a multitude of various different IM protocols - a selection of which are: MSN, Yahoo, AIM, Gtalk and even IRC! Uniquely, with Pidgin you can log in to multiple accounts simultaneously. This means that you can be chatting with friends on AIM, flirting with contacts on Yahoo! - all whilst nattering away to your MSN buddies - all at the same time. Pidgin supports the usual IM features such as file transfer, customised status/nick names, away messages and typing notification. 'Simple' and 'Clean' are two words i would use to describe Pidgin's interface. from http://www.tuxmachines.org/node/47484