I just heard of Cloudo, an “internet operating system”. (Although this name is, strictly speaking, not incorrect, I'll call it a “web operating system” for clarity's sake.) There is no public beta of Cloudo yet, but have a look at this article to get the idea. It really looks much like a desktop OS (OS X, actually), except that it runs in your browser.
Also, Cloudo currently lacks a web browser. Nuff said.