I'd heard whispers about User-mode Linux, but I'd never checked it out for myself. User-mode Linux (or UML) is a patch to the Linux kernel which allows launching a new Linux kernel as a process under an existing machine. Thest best way of thinking about this is to think of your main computer running a virtual computer inside of it. Not only does this allow one to run multiple virtual-machines inside a single real computer, but it also allows compartmentalization for control and security. UML allows Linux within Linux.
CoLinux is the analgous solution for Windows. It allows you to launch a virutal linux machine as a Windows 2000/XP process. This could allow you Windows addicts to try out linux without commiting to dual-boot nonsense.