I've been watching the Xen project for a while, and I think it's probably time I tried it. I like to play around with experimental OS's like Plan9 from time to time. I'm also hoping to build some custom Linux kernels for a work project.
Xen is Open Source software that allow you to run multiple OS's concurrently, with little loss of performance. It achieves this by actually requiring the OS to be slightly modified to work with Xen. As a benefit, any OS can take advantage of Linux device drivers, as well. As a side effect, it means no repartitioning is necessary to try a Xen-compliant OS.
Unfortunately, MS won't let the Xen-ified WindowsXP be distributed, but other OS's are coming around. The Linux 2.6 kernel is already ported, and some distros are starting to bundle Xen in now.
It will be interesting to see if this is a fad or the way of the future.