WindowBlinds - WindowBlinds is a computer program that allows users to skin the Windows graphical user interface.
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.
Cairo Shell - Cairo is a desktop environment for Windows.
Openbox - Openbox is a highly configurable, next generation window manager with extensive standards support.