EyeOS is an open source project to create a browser-based OS that you can host on your own server(s).
It is mainly written in PHP, XML, and Javascript. It acts as a platform for web applications written using the eyeos Toolkit. It includes a Desktop environment with 67 applications and system utilities. It is accessible by portable devices via its mobile front end. eyeos lets you upload your files and work with them no matter where you are. It contains applications like Word Processor, Address Book, PDF reader, and many more developed by the community.
It is already a fairly full-featured system, but there’s a nice roadmap for a January 2010 2.0 release that will include a nice-looking collaborative document editing feature among other things.