i3 - A dynamic tiling window manager designed for X11, inspired by wmii, and written in C.
Wayland - Wayland is intended as a simpler replacement for X, easier to develop and maintain.
Fluxbox - Fluxbox is a window manager for X that was based on the Blackbox 0.61.1 code.
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.
Openbox - Openbox is a highly configurable, next generation window manager with extensive standards support.
XQuartz - The XQuartz project is an open-source effort to develop a version of the X.