The best open source alternative to Wikipedia is Kiwix. If that doesn't suit you, our users have ranked more than 50 alternatives to Wikipedia and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Wikipedia are WikiReader, Wikiless, Citizendium and XOWA.
Provides offline access to Wikipedia and HTML content via ZIM format, featuring a full-text search engine, multi-language interface, export capabilities, bookmarks, content manager, and support for Windows, macOS, Linux, Android, USB drives, and local servers.
Citizendium, powered by MediaWiki, is "an open wiki project dedicated to creating a free, comprehensive, and reliable repository of structured knowledge". Unlike with Wikipedia, a contributor must register using their real name and verify their identity.
XOWA is the free, open-source application that lets you download Wikipedia to your computer. Access all of Wikipedia offline -- without an internet connection!
Oppia is a free, open-source learning platform. Full of interactive activities called 'explorations', Oppia helps curious minds learn and teach.
Welcome to WikiIndex, a wiki of all wiki, wiki people, wiki software, wiki farms, and wiki ideas, a 'work in progress' founded in 2006.
WikiSpooks is a global alternative encyclopedia on politics, spy craft, deep state intelligence, and conspiracies, free of corporate influence, partisan politics, or government propaganda for historians and truth-seekers to read, edit, and share.
LibreGameWiki the free gaming encyclopedia for games developed as open source and using open media.
The Encyclopedia of Mathematics wiki is an open access resource designed specifically for the mathematics community. The original articles are from the online Encyclopaedia of Mathematics, published by Kluwer Academic Publishers in 2002.
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It's a great tool for accessing such content offline, but not a true alternative. More of a compliment