Kiwix - Kiwix enables you to have the whole Wikipedia at hand wherever you go!
XOWA - The free, open-source, offline wiki application
Evopedia - Evopedia is an offline Wikipedia viewer. Features: no Internet connection needed
Wikipedia - Wikipedia is a free online encyclopedia, created and edited by volunteers around the world and hosted by the Wikimedia Foundation.
Aard Dictionary - Dictionary and offline Wikipedia reader.
Infogalactic - The planetary knowledge core