This guide to IM clients over at DigitalTrends goes through all of the individual protocols, but it fails to mention the two apps for IM that I consider indispensable: Trillian & Gaim. These days, almost everyone has more than one IM connection, & you'd be a fool to run separate clients for each of them...

"Though history traces IM’s origins back to Internet Relay Chat (IRC), it is very different now with new protocols, video messaging, file sharing, and more."