Kiwix - Kiwix enables you to have the whole Wikipedia at hand wherever you go!
Wikipedia - Wikipedia is a free online encyclopedia, created and edited by volunteers around the world and hosted by the Wikimedia Foundation.
XOWA - The free, open-source, offline wiki application
Miraheze - Miraheze is a wiki farm (hosts wikis) for free and with no ads, it also provides custom domains...
Infogalactic - The planetary knowledge core
Oppia - Oppia is a free, open-source learning platform.