It's like a virtual OS that can run simultaneously with Leopard. Although, you need an Intel processor.
VMware: Virtualization via Hypervisor, Virtual Machine & Server Consolidation - VMware
I use it for Windows XP which is pretty good since I only have 1GB of RAM so I loaded XP with 256MB.
Here's a screenshot.
Also, a neat feature about this is that you can copy files between OS's.
There is also a similar product to this (and commercial) and that is Parallels but VMWare is much better!
If you want a free software like this, you can try Virtual Box
(or try the VMWare 30 day trial).