This is way geeky cool:
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…
The coLinux site also links to a project called ReactOS, which is an open source effort to create a Windows-compatible operating system from scratch.
Interesting comment about coLinux. Is it possible to run multiple virtual machines?
Not at this time, but it’s supposedly not hard to add (just add reference counting). I’m sure it’s planned for a future version.