Our team simply uses IRC over freenode. Extremely simple, a multitude of great clients (<a href="http://www.codeux.com/textual/" rel="nofollow">http://www.codeux.com/textual/</a>), and completely multi-platform. Most of all, it's free.
We've found Skype group chat feature is the simplest to use, and also shallow on the curve since everyone already has an account.<p>The main issue is the lack of an API to do cool things like integrate with jenkins, and other nifty things that hipchat etc. does.
<a href="https://hall.com" rel="nofollow">https://hall.com</a> also supports WebRTC based video chatting and private channels.
GitHub hooks have been implemented but as of yet there isn't a
proper web API available.<p>It definitely seems interesting though.<p>Edit: if something slick on top of vanilla IRC would be sufficient you could check out <a href="https://grove.io" rel="nofollow">https://grove.io</a> as well.
HipChad is better than campfire in a number of ways. push and email notifications, mobile clients. Easier to have group rooms and to send group messages. lots of plugins.<p>Mobile clients are not great but are good enough.
For our internal use, we use HipChat. We still use Skype occasionally, but it's primarily for external contractors. HipChat ticks all the boxes for us, so give it a try.
KandanApp , the best opensource alternative for HipChat <a href="https://github.com/kandanapp/kandan" rel="nofollow">https://github.com/kandanapp/kandan</a>