Wayland - Wayland is intended as a simpler replacement for X, easier to develop and maintain.
i3 - A dynamic tiling window manager designed for X11, inspired by wmii, and written in C.
Mir - The purpose of Mir is to enable the development of user interfaces shells.
dwm - dwm is a dynamic window manager for X. It manages windows in tiled, monocle and floating layouts. All of the layouts can be applied dynamically, optimising the environment for the application in use and the task performed.
XQuartz - The XQuartz project is an open-source effort to develop a version of the X.
Openbox - Openbox is a highly configurable, next generation window manager with extensive standards support.