Witch - Welcome to the world of W. i. t. c. h.
Wayland - Wayland is intended as a simpler replacement for X, easier to develop and maintain.
Hyperswitch - HyperSwitch provides a compelling alternative to HyperDock for keyboard junkies. What's New
Mir - The purpose of Mir is to enable the development of user interfaces shells.
Contexts - Switch between application windows effortlessly — with Fast Search, a better Command-Tab, a Sidebar or even a quick gesture. Free trial available.
XQuartz - The XQuartz project is an open-source effort to develop a version of the X.