MediaWiki is free server-based software which is licensed under the GNU General Public License (GPL). It's designed to be run on a large server farm for a website that gets millions of hits per day. MediaWiki is an extremely powerful, scalable software and a feature-rich wiki implementation, that uses PHP to process and display data stored in a database. Pages use MediaWiki's wikitext format, so that users without knowledge of XHTML or CSS can edit them easily. When a user submits an edit to a page, MediaWiki writes it to the database, but without deleting the previous versions of the page, thus allowing easy reverts in case of vandalism or spamming. MediaWiki can manage image and multimedia files, too, which are stored in the filesystem. For large wikis with lots of users, MediaWiki supports caching and can be easily coupled with proxy server software.
Engagement Commons Discover apps to engage your community
keep track of edits with watch lists, recent changes pages, and side-by-side "diffs" tool
anti-spam features such as domain name blacklist
multiple "skins" to customize presentation and aesthetics of wiki site
multi-language support including inter-language links
full text search among many organization tools including automatic table of contents generation