Fluid Nexus is another distributed, peer-to-peer messaging technology. This one is implemented as a smartphone app, and uses multiple communication protocols as available:
Fluid Nexus works on an opportunistic, broadcast model whereby it searches for any other device in its vicinity via network modalities that must be explicitly enabled by the user: Bluetooth, and link-local connections over WiFi via zeroconf service discovery. No messages are sent if you are only connected to the Internet via 3G/EDGE.