Mir - The purpose of Mir is to enable the development of user interfaces shells.
Wayland - Wayland is intended as a simpler replacement for X, easier to develop and maintain.
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.
Xynth Window System - Xynth Window System is a web-based software that is used for the deployment of resources within a minimum resource and offers you features that are alternatives to the X Window System.
Metisse - Metisse is an open-source software solution that is used for a seamless implementation of an interface on your desktop.
X11 - X11 is no longer included with Mac, but X11 server and client libraries are available from the XQuartz project.