i3 - A dynamic tiling window manager designed for X11, inspired by wmii, and written in C.
Contexts - Switch between application windows effortlessly — with Fast Search, a better Command-Tab, a Sidebar or even a quick gesture. Free trial available.
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.
Witch - Welcome to the world of W. i. t. c. h.
Openbox - Openbox is a highly configurable, next generation window manager with extensive standards support.
VistaSwitcher - VistaSwitcher is an elegant and powerful task management utility for Windows OS.