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.
awesome - A dynamic window manager for the X Window System developed in the C and Lua programming languages.
VistaSwitcher - VistaSwitcher is an elegant and powerful task management utility for Windows OS.
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.