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.
DirectFB - DirectFB is a web-based platform that provides you with complete access to a software library that you can use for the acceleration of graphics, handling the input devices, and others for your Linux operating systems.
Metisse - Metisse is an open-source software solution that is used for a seamless implementation of an interface on your desktop.
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.
X11 - X11 is no longer included with Mac, but X11 server and client libraries are available from the XQuartz project.