There are Web 2.0 applications to do almost anything. But I just found out that there does not seem to be any UML Web 2.0 application out there. I must admit I am not a big fan of these types of software engineering techniques, but from time to time, you do have to sketch out the design of your application and UML-like diagrams are a sane way to communicate your idea. But UML, almost by design, is meant to be a shared view of your design. This makes it a prime candidate for Web 2.0.
Sounds like a good start-up project.
Update. Eran accurately points out that Gliffy may do the job. Indeed. Nevertheless, there is still, I think, lots of open room for Web 2.0 applications having to do with programming. I’d love to do an entire non-trivial programming project using only Web applications (excluding any desktop application other than my browser).