MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia. It is now used by several other projects of the non-profit Wikimedia Foundation and by many other wikis. MediaWiki is 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, such as MySQL.
DokuWiki is a standards compliant, simple to use Wiki, mainly aimed at creating documentation of any kind. It is targeted at developer teams, workgroups and small companies. It has a simple but powerful syntax which makes sure the datafiles remain readable outside the Wiki and eases the creation of structured texts. All data is stored in plain text files, no database is required.
PmWiki is a wiki-based system for collaborative creation and maintenance of websites. PmWiki pages look and act like normal web pages, except they have an "Edit" link that makes it easy to modify existing pages and add new pages into the website, using basic editing rules. You do not need to know or use any HTML or CSS. Page editing can be left open to the public or restricted to small groups of authors.