KwikiWiki
Kwiki is perhaps the simplest, most modular, and easiest to extend Wiki. A Wiki allows users to freely create and edit web pages in any web browser. Kwiki is Open Source Software written in Perl, and is available on CPAN.
My interest in developing the Kwiki is in maintaining UTF-8 support for international users. Most recently, I submitted patches for Kwiki31UnicodeFixes. When I have time, I plan to port my formatter modules to the new subclass system.