Codeuml.com is a web based UML designer where you code the diagram using a special language and it generates the diagram on the fly. It is faster than using any visual designer where you have to drag & drop diagram elements and use mouse to connect them. Codeuml uses the open source plantuml engine to produce diagram from text. You can produce UML diagrams as fast as you can code.
This web application shows some interesting design and coding challenges. First, it shows you how to build a web based IDE like environment that mimics Windows 8 Metro UI. Second it shows how you can periodically collect data from the website, send to the server in the background asynchronously and get the result generated on the fly. Third and the most important, it shows how you can maintain a server side pool of very expensive resource that you cannot just create on every hit to the server and must have a finite pool that is shared by all your web users.
Read details about this from this CodeProject article: