After reading a bit about, Paper Airplane, my first impression is that it sounds a bit like Groove, but differs in that it’s integrated into the browser (Mozilla/FireFox currently) and built on-top of the Java JXTA and P2P Sockets framework. I haven’t tried it yet, but it seems worth a look even in its early beta state.
“Paper Airplane is a Mozilla plugin that empowers people to easily create collaborative communities, known as Paper Airplane Groups, without setting up servers or spending money. It does this by integrating a web server into the browser itself, including tools to create collaborative online communities that are stored on the machine. Paper Airplane Groups are stored locally on a user’s machine. A peer-to-peer network is created between all of the Paper Airplane nodes that are running in order to resolve group names, reach normally unreachable peers due to firewalls or NAT devices, and to replicate content.”