MediaWiki : No.1 Free Wiki Software

MediaWiki is a free wiki software package that can be used to install Wikipedia like collaboration environment on any website or server. If you’re looking for a flexible wiki Content Management System then MediaWiki is the package you need. It can be used for a wide variety of projects, regardless of the amount of people involved.

Whether it’s a solo writer writing a book or a 100 people team working on a massive assignment, MediaWiki can handle it. It’s simple to install, very easy to learn, and absolutely free.

Some Features

Collaborate with as many people as you want.
Extendable: Install extensions for additional functionalities (add calendar, integrate chat, add backup /restore functionalities, clipboard, etc.). Hundreds of extensions available.
Fullly configurable: change look and skin, restrict certain pages, add categories, enable / disable file uploading, and lots more.
Support: Well documented. So if you want to add something to your wiki but have no clue how it can be done just search for it on MediaWiki.org

Used by over 50 popular online websites (incl. Wikipedia). Check out the full list here.