Crazy idea, but it's amazingly cool!   Here's an IDE that supports C#/Silverlight, ASP.NET, straight JavaScript, and PHP.   It's amazingly full-featured and runs completely online.   You can try it out without even registering, or you can register for free to save your projects.   Think of it as CodePlex/SourceForge for online apps on steroids!   I haven't played with collaboration, and I don't think it does version control, but as a single-user app it's really good.   Definitely something to keep an eye on.

Link: http://coderun.com/ide/