Here's the video demo I promised earlier on Startforce, a Web OS that's not yet in open beta in the United States. While not a new idea, this implementation is especially clean. It apes the Windows look and feel, which may not be a great thing from an aesthetic perspective, but it does mean that the system is very easy for new people to get in to.
I find products like this technically stunning, yet it's still an open question as to whether we actually need them. Developers won't build apps for platforms that don't have big user bases. That's why Facebook is brilliant: The company built a large and interconnected user community before it launched its open platform. Furthermore, other technologies may solve the problems that Web OSes address. Online storage services such as Box.net are getting APIs so other Web apps can use them for integrated storage, and OpenID (or competing services) may solve the hassle of having to log in to each service you use separately.
Nonetheless: this is cool.