Firechat is an "off-the-grid" messaging service that allows you to message people without an Internet connection.
Firechat makes use of both Apple's multi-peer and Android's mesh technology to create a cross platform Internetless network. The more people using it, the greater the strength of the network.
Firechat also has on-the-grid anonymous messaging and chaotic chatrooms that support up to 10,000 people.
It's an impressive app, but it still needs some fleshing out before it's practical.