Wikipedia is a free, collaboratively edited, and multilingual Internet encyclopedia supported by the non-profit Wikimedia Foundation.
MoinMoin is an advanced, easy to use and extensible WikiEngine with a large community of users. Said in a few words, it is about collaboration on easily editable web pages. MoinMoin is Free Software licensed under the GPL - living from lots of people contributing
Wagn is an open web platform for building coherent web systems.Using simple wiki-inspired building blocks called cards, “Wagneers” construct sites ranging from simple content managers to custom project trackers to fully integrated web suites.
MediaWiki is a free software open source wiki package written in PHP, originally for use on Wikipedia. It is now also used by several other projects of the non-profit Wikimedia Foundation and by many other wikis, including this website, the home of MediaWiki.