Cooperative Linux is the first working free and open source method for optimally running Linux on Microsoft Windows natively.  
More generally, Cooperative Linux (short-named coLinux) is a port of the Linux kernel that allows it to run cooperatively alongside another operating system on a single machine. For instance, it allows one to freely run Linux on Windows 2000/XP, without using a commercial PC virtualization software such as VMware, in a way which is much more optimal than using any general purpose PC virtualization software.
Good link
deathburger: And these kittens think so too.

One question
Timmy: Can it run WINE?  
deathburger: You're a funny guy.. :)
parmentf: Exactly what I thought. Perhaps even a bit perverse ;)