I second ultramon. Its the only program I've ever downloaded and liked enough to actually pay for when the trial ran out. Its slick, well designed and very easy to use.
if you have an ATi card or something with an ATi core usually in the software it comes with a program called Hydravision and that is what i use and it works amazing
I think you're looking for software to handle Virtual Desktops and I think people are confusing that with multiple desktops with multiple monitors. I don't know of any free software that will do it outside of a Litestep or similar shell, but there's a list of shareware vitual desktop managers here