Qubes is an open source operating system designed to provide strong security for desktop computing. Qubes is based on Xen, X Window System, and Linux, and can run most Linux applications and utilize most of the Linux drivers. In the future it might also run Windows apps. [more]
Architecture page here.
Qubes lets the user define many security domains implemented as lightweight Virtual Machines (VMs), or “AppVMs”. E.g. user can have “personal”, “work”, “shopping”, “bank”, and “random” AppVMs and can use the applications from within those VMs just like if they were executing on the local machine, but at the same time they are well isolated from each other. Qubes supports secure copy-and-paste and file sharing between the AppVMs, of course.