Firestr is an interesting prototype of a P2P messaging program. It’s encrypted and Free Software. But the really cool feature is that people can not only load various plugins that extend the functionality, they can code new plugins directly within the app (using Lua) and do so even collaboratively, with their chat partners, and they can distribute their plugins from within the app to their contacts.
It would be really interesting if someone could code things like distributed newsgroups, filesharing or a distributed microblogging network (where you can follow even non-contacts) using this system. The author says that upcoming versions will have support for fully distributed data structures that will make this much easier.
This could be a replacement for RetroShare which — while conceptually interesting and having an active community — never went mainstream (in my opinion, mainly for reasons having to do with usability, no security reviews and a really outdated UI).
If anyone has a firestr account and wants to chat, send me an email with your identity.