Sysinternals Desktops: http://technet.microsoft.com/en-us/sysinternals/cc817881

mdesktop: http://code.google.com/p/mdesktop/

Dexpot: http://dexpot.de/index.php?lang=en

Virtual Dimension: http://virt-dimension.sourceforge.net/

VirtuaWin: http://virtuawin.sourceforge.net/

Finestra: http://vdm.codeplex.com/