Contexts - Switch between application windows effortlessly — with Fast Search, a better Command-Tab, a Sidebar or even a quick gesture. Free trial available.
i3 - A dynamic tiling window manager designed for X11, inspired by wmii, and written in C.
Witch - Welcome to the world of W. i. t. c. h.
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.
Hyperswitch - HyperSwitch provides a compelling alternative to HyperDock for keyboard junkies. What's New
Openbox - Openbox is a highly configurable, next generation window manager with extensive standards support.