Spark (client) and OpenFire (server)
(using XMPP / Jabber)
office communicator (msn of some type)
Technically we have Sametime (part of Lotus Notes) available, but I don't use it since a) Lotus Notes blows, and b) most people I have to talk to are sitting pretty close by, so I might as well just walk over and talk to 'em if I need to.
