Friki is a Java Wiki. It is simple to deploy (just drop in to any modern servlet container), small and very easy to customise after deployment. Supports the classic wiki markup and a few common extensions.
Well written wiki, however the stringtree.jar library, which is the heart of the wiki, has no available source code or documentation and the stringtree project in sourceforge is incompatible with the one used by Friki. Developers are unavailable to respond so it's not possible to completely build from source and extend.