website engine for collaborative work

 MediaWiki is a wiki engine (a program for creating a collaboratively
 edited website). It is designed to handle heavy websites containing
 library-like document collections, and supports user uploads of
 images/sounds, multilingual content, TOC autogeneration, ISBN links,
 etc.
 .
 Moreover, it keeps track of changes, so users can receive
 notifications, view diffs and revert edits. This system has many
 other features and can easily be extended.