VMWare Player is free (as in $). When last I used it, one could not use it to create a new VM. There are many available for download, and there are online utilities to do so.
VirtualBox by Sun/Oracle is free (as in source code).
I just installed VirtualBox on my Win7 machine last night and installed Ubuntu 9.10 64 bit on it. Quite pleased thus far. On top of all that I installed DeskSpace to give my Windows machine virtual desktops (like Linux and Mac already have and have had for years). I put my virtualized Ubuntu on one desktop in full screen mode and I can easily switch between it and windows with a quick flick of some keys.
I heartily agree with using a virtual desktop manager, and placing the client virtual OS on one of the desktops and then switching between the client OS(es) and Windows. It still amazes (or is that appalls?) me that Windows does not include a virtual desktop manager.
I use AltDesk, not as spiffy as the DeskSpace cube, but serviceable (it was the best option I could find when I first searched for virtual desktop managers about 7 or 8 years ago, and I have gotten all of my updates for free since I originally purchased it back then).