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').
Y Window System - Y Window System is a platform that allows you to improve the speed, working, and efficiency of the application in your operating system and helps you to increase the responsiveness of applications similar to any locally based app.
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