Jabber, according to Gaim (which says it's quoting jabber.org), is the coolest IM system on the planet. It is a decentralized, open, XML-based protocol; moreover, it has a Unix-like sense of humor; from the Jabber user guide:
There are a lot of pieces here: clients running on different operating systems, multiple servers, a communication channel between the servers, and two star-crossed lovers. Jabber handles everything but the last part [...]
Most importantly, of course, Jabber has a proposed protocl extension called Presence Obtained via Kinesthetic Excitation - POKE for short. So, poke me on AIM for my Jabber account name.