YouOS is an Ajax-based experimental "Web operating system" creates by guys at MIT. It is a window-based operating system with GUI that runs on browser so you can access your desktop from any computer anywhere. YouOS tracks a user's state across user sessions, so after you log off, you can log on from anywhere and your desktop and applications will be in the same state where you left them.
Amazingly, there are already nearly 200 applications you can install on YouOS account - rich text editor, email client, chat, RSS reader, also a couple of different web browsers! It's funny that the OS runs on browser and has its owen web browser. The screenshot show my other blog site displayed on YouBrowser. I dunno if it is really useful to view web within web though.