Wikidia is a hybrid of diagramming tool and wiki. You can create good-looking UML diagrams with it in no time. Because underneath it's just human-readable text, you can edit them easily and rapidly, you can put them into the source control, merge them or do any other text-based operation.
At this point Wikidia is just an experiment though. Its goal is to create a prototype which would validate that it is possible to build such tool using JavaScript and SVG and in the process help me to earn my degree :-).
Disclaimer: this is a prototype-quality code. It contains many TODOs, bugs and half-baked ideas.