RainingCubes is a screen saver for OS X that animates anywhere from one to thousands of multi-colored cubes doing what cubes do when they have no surface beneath them, and the force of gravity is applied to them.
That might not be too interesting on its own, but RainingCubes is the first screen saver for OS X that does its drawing using the new Metal 3D drawing API in OS X 10.11 (El Capitan).