CodeTek VirtualDesktop Pro/Lite
CodeTek just released Virtual Desktop 3.0 which now comes in two flavors, Lite and Pro.
There are other virtual desktop apps for Mac OS X out there, but CTVD is definitively the best one. It's also the most expensive (as others are free) but I think it's well worth the price. I can't live without it, anyway! Also, CodeTek has often shown that they listen carefully to their users, and after almost two years doing beta testing for them, I can say that great people work there. These are two other good reasons to support them!