The one I use is http://virtuawin.sourceforge.net
It's free, open source etc.
It works on Windows 7 64bit and goes down to Windows XP /Windows Server 2003 (as far as I got to test).
It doesn't have a fancy desktop cube or anything but is easy to use and the only visual thing (apart from switching desktops) is a little taskbar icon
Alternatively you can use Dextops which offers a lot of flexibility and while it may not be as pretty as something like Deskspace it does more for adding efficiency to how you organize your workspace. Thre are a couple other options like Cubedesktop, I found a review of all 3 here: http://blog.netflowdevelopments.com/2012/09/19/3d-desktop-cube-reviews-and-comparisons-windows/