Bitchat

Bitchat is a chat app that works without the internet. Nearby devices link over Bluetooth into a mesh, and messages hop directly between them rather than through a server. Group rooms and one-to-one messages both work.

It borrows the feel of IRC, and the screen stays channel-shaped to match. The situations it is written for are the ones where connectivity fails first: a trail with no bars, a remote area, a community that has to keep talking anyway. Range, in exchange, is whatever Bluetooth and the density of nearby devices allow.