We first noted the open-source virtualization application VirtualBox way back in 2007, and since then this open-source competitor to VMware Fusion and Parallels has come a long way. Most importantly, perhaps, the project was acquired by Sun Microsystems and has now become Sun xVM VirtualBox with a great deal more support.
The application is free for personal use and is available for a wide variety of x86 platforms including OS X on Intel Macs. The feature set is extensive and includes a lot of the features of its commercial counterparts. That said, it lacks many of the niceties of those programs, including any kind of 3D graphics support. Nonetheless, it's hard to argue with free and is probably worth checking out if your virtualized Windows requirements are modest.
Sun xVM VirtualBox is a free download from Sun.