Jamie just pointed me to YouOS, an operating system running entirely within a web browser:
YouOS and its applications run entirely within a web browser, but have the look and feel of desktop applications. An application’s code and data reside remotely but are executed and modified locally. This model allows for a great deal of freedom. You can edit a document at home in a text editor and then go to school or work and instantly access the same text editor and document.
You can even open a web browser and surf inside YouOS…