https://slack.com/
Why not? I know that some of you use IRC, I remember the jacoders of the openJK one, but I was thinking, I'm on the Cryengine slack, one for Windows sysadmins and other slack for work, and I'm convinced that is an awesome tool for team communications.
I think that an slack for the entire JK community (not just jk2.info, jkhub.org too and even massassi.net) would be a great idea.
Just suggesting.