CoLinux

Cooperative Linux (coLinux) is the first working free and open source method for optimally running Linux on Microsoft Windows natively. More generally, Cooperative Linux 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.

It began with running the KNOPPIX Japanese Edition on Windows, and now has many other Distributions.