TWiki - Open Source Wiki

Twiki is a flexible, powerful, and easy to use enterprise wiki, enterprise collaboration platform, and web application platform. It is a Structured Wiki, typically used to run a project development space, a document management system, a knowledge base, or any other groupware tool, on an intranet, extranet or the Internet. Users without programming skills can create web applications. Developers can extend the functionality of Twiki with Plug-in. Twiki fosters information flow within an organization, lets distributed teams work together seamlessly and productively, and eliminates the one-webmaster syndrome of outdated intranet content.
Twiki has been downloaded over 500,000 times and is used daily by millions of people in over 100 countries. Some larger deployments have several 100,000 pages and over 10,000 users.