Chrome OS, announced by Google earlier this year, is promised to be an open-source, Linux based OS that will run on both x86 and ARM based systems. It is going to be based on the Chrome browser and will be initially targeted at low-powered netbooks, with support for higher end systems coming later.
All details on how to download it and install it here : http://www.techcrunch.com/2009/11/19/guide-install-google-chrome-os/