Qubes uses virtualization, sandboxing to deliver a more secure desktop OS
Qubes is a bit different from the desktop OSes most of us are familiar with, though it looks similar. It's built on Fedora and Xen and aims to deliver a more secure desktop through the use of sandboxing and "App VMs." Each VM is isolated, though you're still able to do pass data between them (via secure copy and paste).
There's a price to pay in terms of system resources for the added security, but it's reasonable. A typical App VM uses about 400MB of memory but can run a handful of apps in that space (e.g. Firefox, Thunderbird, and OpenOffice).
Makes me wonder...With XP mode already available in Windows 7 and the virtualization improvements coming to SP1 and sandboxing becoming more and more common, maybe we'll see something like this in Windows 8. Hey, it's a possibility.