#Kiwix is an app that lets you easily browse wiki and stackexchange sites offline.
It's extremely useful for people who cannot access a reliable internet connection for whatever reason. (For example, some sailors will download a copy of wikipedia onto Kiwix in port to browse it while at sea.)
Kiwix is available for Windows, Mac, Linux, Android and iOS:
You can also make a local Kiwix server on a Raspberry Pi: