Virtual Box

I used to use VMWare but switched to VirtualBox when VMWare 2 came out. It's not as Mac-like as VMWare (as it uses Qt instead of Cocoa) and it's not as fast, but it's free and it's fast enough to do everything except demanding 3D programs (it does support OpenGL 1.5).

They have a page that can help you decide (though it's a little outdated).

http://www.virtualbox.org/wiki/VBox_vs_Others

I read about it today at AppleInsider and downloaded, installed it, created a VM and installed XP in about 90 minutes.

I like.

I created a shared folder so I can share files between OS/X to XP and installed a few Windows programs to see how well they worked as well.

All is good.

It looks like I have to mount/unmount the dvd-drive in the menus when I want to use it in XP or make it available in OS/X again. And it shows the Windows taskbar no matter what which is a bit annoying so I moved it to the side of the screen.
From the sound of it, the competition is more advanced, but the competition isn't free. And VB makes a solid first impression.