If you haven't read about it on Slashdot, Jabber is an open, extensible instant messaging system. I've known about it for some time, as a friend was writing a similar system, but stopped because it would be a duplication of Jabber.
Anyhow, why should you give it a shot? First of all, the protocols are all completely open, so anybody can make Jabber servers or clients. Second of all, they have gateways for AIM, MSN, Yahoo!, and ICQ so that you can just use *one* program to talk with *five* sets of users. Third, Jabber uses a distributed system so if one server goes down, only the people that use that server are affected. If the AIM server goes down, nobody can use AIM at all.
Of course, there are a few problems. The most noticeable problem is that the current clients aren't the most stable programs I've used... Also, the servers don't always behave properly, so sometimes things won't work. Overall though, it's quite usable.
btw, I'm email@example.com. Hope to see you online sometime. :)