Ulteo today unveiled their Virtual Desktop which is a free, full Linux desktop that runs seamlessly on Windows. It's interesting because it's not running under Xen or VMWare, but instead uses the coLinux patch, which they claim allows the system to achieve 'great performance, close to a native installation on the PC.' No need to reboot the system anymore to switch from Windows to Linux.

http://www.colinux.org/
http://www.ulteo.com/