No doubt you’ve heard about [Apple’s Boot Camp Beta](http://www.apple.com/macosx/bootcamp/) – which enables you to boot either Mac OS X or Windows XP on a Intel-based Mac.
What if you want to run both simultaneously?
You need virtualization!
> Virtualization makes it possible to install Windows XP, Linux, OS/2 Warp and other Intel-compatible operating systems on Intel-Mac hardware and run the alternate operating systems from the comfy confines of OS X.
Wired has [published an article](http://www.wired.com/news/technology/computers/0,70604-0.html) which explains it with links to products out there.