Wiki visualization, and InterWiki visualizations are quite interesting. All though there are a few implementations of it, none of them are sufficient to actually replace the browser as a read/write tool. They are an interesting tools to browse data with though.
TouchGraph (Java) - OpenSource
Kartoo (Flash) A visual search engine, if you seach specifically for "wiki" as the first search term it seems to put you into a wiki specific view.
Silvertejp (Java) - An UML editor which uses MoinMoin to generate GraphViz maps.
Silvertejp really has nothing to do with MoinMoin, yet. But I've had MoinMoin in mind while creating the ASCII UML notation. --KarlWettin