Wakka is a PHP based WikiEngine designed for speed and easy extensibility. Because of its rich descendancy, it has been nominated as one of the best wiki engines in the WikiEngineHallOfFame. http://www.wakkawiki.de/
As of March 2004, it seems that WakkaWiki is no longer being actively developed. This is too bad, because it was a very promising Wiki. However, the good news is that there are many forks. Unfortunately, none of them seems to have much in the way of English documentation, one doesn't have code yet, and another is only free for personal use.
As the developer said, he stopped the development of WakkaWiki. It is true, that the documentation of the forks could be better, but it is improving (November 2004).
As of August 2005, two Wakka forks offer extensive documentation in English, both for end users and developers.