I've been waiting for years to have an IM app that I could use with the other developers on my team(s) to communicate about projects and integrate updates from source control and bug tracking. I'm not too keen on using proprietary apps or subscription-based apps in most situations but Hipchat has far exceeded my expectations in the few weeks that I've been using it.
Besides having a slick interface and the features that I need it uses XMPP/Jabber and has both desktop and web versions of the app (written using Adobe Air). One of the features that I really like is when someone uses your username with an @ symbol preceding it and you're offline you'll receive an email with the message.
We've been using this primarily to have a record of our debates, conversations, and to have an ongoing record of important tidbits, URLs, git commits, etc. It also has been helpful for having a log of our planning discussions which we can use when bringing new developers onboard to let them sort of get into our heads and see how the app developed to its current state.
Go checkout Hipchat and signup for their free 30-day trial.