twiki - http://twiki.org/
Welcome to TWiki, a flexible, powerful, and easy to use enterprise collaboration platform and knowledge management system. - [Read more]

xwiki - http://www.xwiki.org/xwiki/bin/view/Main/WebHome
xwiki is an open-source project focusing on developing a Java eXtended Wiki engine. - [Read more]

riki engine - http://smart-engines.dyndns.org/riki/
Riki is a wiki built in Ruby on Rails. The main focus is project management on intranets, but can be used in any collaborative environments. - [Read more]