A wiki is a page or collection of Web pages designed to enable anyone who accesses it to contribute or modify content, using a simplified markup language. Wikis are often used to create collaborative websites and to power community websites. The collaborative encyclopedia Wikipedia is one of the best-known wikis. Wikis are used in business to provide intranets and Knowledge Management systems. Ward Cunningham, developer of the first wiki software, WikiWikiWeb, originally described it as "the simplest online database that could possibly work".
http://en.wikipedia.org/wiki/Wiki

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...

MediaWiki is the collaborative editing software that runs Wikipedia, the free encyclopedia, and other projects. It's designed to handle a large number of users and pages without imposing too rigid a structure or workflow.

PmWiki is a wiki-based system for collaborative creation and maintenance of websites.

BoltWire is a small but easy to use web development engine with surprizing flexibility and muscle. If you want lot's of power, but an easy learning curve–BoltWire might be right for you! Best of all it's free...

AWS XMS is an online visual web development enviroment and framework, providing a web application base, with multi language support, based on XML

Open source software combining the best of wiki and CMS with community building-features.