coWiki

From the front page:

coWiki is a free (GPL) tool written in PHP 5 which creates a collaborative website that gathers, organizes and extends knowledge. It can manage text documents and binaries, act like a weblog or organize knowledge bases such as FAQs.

coWiki provides an easy way to maintain and discuss its documents. It also provides a fast way for qualified contributors to create content online, using a simple and easy to apply markup language without needing HTML editing skills.

coWiki is template based, multilingual and modular.