I have recently discovered the very cool world of VMware! I’ve been playing with virtual machines running a bunch of different operating systems: fedora, gentoo, debian, freebsd, ubuntu, mandriva
I know those are mostly just linux flavors, but you can run DOS, Windows 3.1/95/NT/2000/XP/VISTA!, etc.
Check it out:
http://www.vmware.com/products/free_virtualization.html
The VMware Server is way cooler than the player. You can make your own virtual machines. Oh, and it’s all FREE.
VMware has a directory of images:
http://www.vmware.com/vmtn/appliances/directory/
and here is another site with some good VMware images:
http://www.thoughtpolice.co.uk/vmware/