Ricochet
With recent events it is very obvious that LE is taking a much more drastic approach to silence us.
In the case of a completely anonymous IM service most suggest XMPP using OTR however I'd suggest taking a peak at a totally anonymous option called Ricochet, it only connects to tor and only assigns usernames via a tor hidden service address.
This uses no servers meaning no logs it has been audited and granted as one if not the most secure p2p messaging service to date.
If you feel like taking a glance at the source code here it is.
Thanks fed, I'll keep your secure website in mind ;)