MediaWiki is the collaborative editing software that runs Wikipedia, the free encyclopedia, and other projects. It's designed to handle a large number of users and pages without imposing too rigid a structure or workflow.
Many available extensions or plugins available. Custom programming may not be necessary.
Management of content and data is relatively straight forward, but there may be a bit of a learning curve. Some programming experience may be required.
Has a smaller active community. Limited support is available either for free or as a paid service. Adequate documentation is available.