Hi HN,
I decide make our product GeniusWiki free since version 2.0. GeniusWiki is a Wiki engine. It aims to provide an easy way to organise knowledge, ideas and documents etc. It is the software used for http://www.geniuswiki.com which is free sign up website. You can download and install GeniusWiki to your owned server for internal usage. You only need to take 10 seconds to fill a form for a unlimited user license request.
GeniusWiki is base on Java and OS independent. It supports most popular database and Java web server. We make a lot efforts to make GeniusWiki ease-to-use and ease-to-maintain. It has a setup wizard to guide user to complete install. It has both WYSIWYG and powerful markup editor and switch between seamless. Except the better writing experience, it has lots handy feature, such as the fine-grind and flexible permission control, conflict handling, D&D dashboard, widget, powerful admin platform and etc. GeniusWiki is quite stable for team or personal usage as geniuswiki.com is already running over 3 years and hosting thousands users and spaces.
We make it free just want to make this software more popular. Hope you like it and give us more feedback.