Wayland - Wayland is intended as a simpler replacement for X, easier to develop and maintain.
Mir - The purpose of Mir is to enable the development of user interfaces shells.
XQuartz - The XQuartz project is an open-source effort to develop a version of the X.
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.
i3 - A dynamic tiling window manager designed for X11, inspired by wmii, and written in C.
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.