From The Guardian Project:
An app store, encrypted chat, and more, all available over WiFi from an off-the-grid Raspberry Pi.
How does it work?
Butter Box broadcasts its own WiFi network. Once you join, you can install Butter, get apps, join a public chatroom, or make your own private one to share with friends. Butter Box works even when there’s no Internet. Just stop by the box to download more apps or check in on your chatrooms. Everything is stored right on the Butter Box.