Miraheze - Miraheze is a wiki farm (hosts wikis) for free and with no ads, it also provides custom domains...
Wikipedia - Wikipedia is a free online encyclopedia, created and edited by volunteers around the world and hosted by the Wikimedia Foundation.
Wikiwand - A beautiful new interface to Wikipedia
Oppia - Oppia is a free, open-source learning platform.
Shoutwiki - Free Wikifarm, using MediaWiki software.
Golden - Mapping all human knowledge