MediaWiki is the collaborative documentation software underlying Wikipedia. It is highly extensible and can be self-hosted on a server with standard software. It is administered by the Wikimedia Foundation.
This website also runs on MediaWiki.
Create a class wiki
Students can collaborate on building a shared knowledge base of information they're learning together.
Hold a Wikipedia edit-a-thon
Identify topics that are inadequately covered at Wikipedia.org and organize an event to add material on those topics.
How to use it
The mediawiki.org website, itself a MediaWiki, is full of information about how to use the software.
It can be deployed with Cloudron.
Replace built-in wikis on learning management systems, which are often very limited in functionality if they exist at all.