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. You can download it and try it for free at EyeOS.org .
The only catch is you need PHP and apache webserver running on your machine. I tried this OS and am really impressed. You can set the wallpaper, create user accounts, and more. And what is more, it is platform independent too.