Wayland - Wayland is intended as a simpler replacement for X, easier to develop and maintain.
VirtuaWin - VirtuaWin is a virtual desktop manager for the Windows operating system (Win9x/ME/NT/Win2K/XP/Win2003/Vista/Win7/Win10). A virtual desktop manager lets you organize applications over several virtual desktops (also called 'workspaces').
Mir - The purpose of Mir is to enable the development of user interfaces shells.
Sysinternals Desktops - Desktops allows you to organize your applications on up to four virtual desktops.
XQuartz - The XQuartz project is an open-source effort to develop a version of the X.
9Desks - up to nine virtual desktops