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.
Fluxbox - Fluxbox is a window manager for X that was based on the Blackbox 0.61.1 code.
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.