UmlCanvas is a Javascript library that turns an HTML5 Canvas element into a dynamic and interactive UML diagram editor and presentation tool ... on every HTML page, on and off the web. 
UmlCanvas also offers a hosting service that puts your diagrams online. Design your diagram once, include it in many HTML-based formats: your own pages, your blog, feed readers and even Google Wave. 
Integrating CASE tools and hosted UmlCanvas, creates a collaborative platform that enable modelers to step out of the closed environment of their desktops and extend their modeling into the social web.