OpenWRT

OpenWRT is a /Linux distribution originally targeting Wireless RouTers but for now any suitable embedded devices (/DIY/electronics!).

Often OpenWRT helps to build the "own clouds" (see /self-hosting).

Derivatives

LibraryBox

an open source, portable digital file distribution tool based on inexpensive hardware that enables delivery of educational, healthcare, and other vital information to individuals off the grid.

LibraryBox is a combination of a router (a variety of hardware will work), USB drive, and software that, when combined, give you a small, low powered webserver. The webserver acts like a captive portal, and delivers files that are stored on the USB drive.
- BiblioBox, the "Nomadic device allowing access to digital resources" (G-translated from "Dispositif nomade permettant d’accéder à des ressources numériques"), a fork of LibraryBox. The site is made using /TiddlyWiki.
PirateBox (+ the "/r/PirateBox" subreddit)

a DIY anonymous offline file-sharing and communications system built with free software and inexpensive off-the-shelf hardware.

PirateBox is inspired by the free culture and pirate radio movements.