Links - Wikis and Knowledge Wikis
Updated Oct 10 2008: added TWiki
There are several small, fast and powerful open source wikis that can be used as a full-featured CMS to for building small web sites and are fairly easy to extend and customize. Some may be capable of providing knowledge representation and management functions as well as the usual features available in traditional wikis. They may be the closest existing solution to some of the requirements for a Semantic Web ( however it gets defined ).
The fly in the soup is that learning advanced markup languages can be more formidable than learning a simple Wikipedia-style markup language. They can require a significant learning curve for new users, but the time spent seems to pay off in terms of power and flexibility.
WikkaWiki is a small (2M unzipped ), simple and powerful application based on the venerable WakkaWiki. An {{action}} statement in the wiki page generates a call to a snippet of PHP code - parse some arguments, emit some HTML and "Bob's your uncle". New programmers ask me about the easiest way to learn the basics of PHP/Web development and WikkaWiki may be my new recommendation.
Two articles about integrating WikaWiki with CakePHP.
Another interesting application is PmWiki, which is even smaller than WikkaWiki (1M unzipped ), but is far more ambitious and complex. It's seems to be very powerful - their definition of "skins" may take the idea to a whole new level - skins are not just presentation layer but represent behavioral "skins" as well. Another interest feature of PmWiki is that it is based on flat files rather than a relational database ( mySQL ). A potential scalability issue ? Not sure, still evaluating.