WindowLabWindowLab is a small and simple window-manager of novel design.
From the homepage:
It's click-to-focus, shares its window depth policy with the Amiga, and has a window resizing/reshaping method similar to that of 8½ from Plan 9. WindowLab maintains the illusion of direct manipulation by constraining the mouse pointer when appropriate, ie when a window cannot be dragged any further in one direction. The pointer is also constrained vertically (effectively making the target menu items infinitely tall according to Fitts's law) when it's in the menubar to reduce pointing time.