This is a interesting tool, a full web based UML, and other diagrams designer tool, just like mockupbuilder. you can checkout this tool at http://creately.com/.
You would be surprised to see the client list of this tool. And some surprising integration with your existing collaboration tool.
But for uses we have a simple problem, that’s money proportional to extendibility. This tool has some basic free service but for advanced services you have to use the commercial stuff. Why we don’t have free stuff? its because the world will not give us the basic needs for free so some one have to do business to make money.
Did I mention that you can build your software mockup as well, in this tool apart from doing awesome collaboration. And not to mention it has a desktop version as well, incase you want to replace MS Visio with Creately.
“I dream of a all free software world.”