Yod'm 3D is Yet another desktop manager (DeskSpace of Otaku Software) provides a way to have four desktops on your windows. The four desktops are like walls of a cube when switching . This may enhance productivity of people but I use it for aesthetic purpose mostly.
Virtual Desktop Manager is an offering in Power tools section of XP which has other interesting also like ALT + TAB replacement. This one also has four desktops which are like 2 X 2 grid.
Madotate allows a interesting way to arrange item on your desktop
Topdesk from Otaku software offers ways to switch between your open applications on windows
Flip like Vista
Expose like Mac
As I had mentioned earlier power tools for XP provides ALT + TAB replacement
While talking about desktop managers one cannot forget to mention Compiz for linux.
If you have not seen Compiz in action here are few videos
