Cairo Shell - Cairo is a desktop environment for Windows.
i3 - A dynamic tiling window manager designed for X11, inspired by wmii, and written in C.
bbLean - Alternative Windows shell
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.
BB4Win - BB4Win (Blackbox for Windows) is an alternative shell for Microsoft Windows.
Openbox - Openbox is a highly configurable, next generation window manager with extensive standards support.