Today I happened across the eyeOS project which aims to create a browser based personal content management system. It is PHP based, has partial AJAX support and is GPL. Cool idea, it could be an easy way for web newbies to create and maintain websites.
From the site:
EyeOS is a free, cross-platform Personal Content Manager System based upon the style of a Desktop Operating System. The base package includes the whole Operating System structure and ten apps, as a Calendar, a File Manager, a Text Editor, an Internal Messenger, a Browser and a Calculator.
I’ve also done a small article about EyeOS over at my page, http://www.heavysheep.com/content/view/50/2/ . And if you are interested, you should check out High Earth Orbit, from here http://www.heavysheep.com/content/view/54/2/ . They have some more information about web-based OS.