Unlike computer mice, trackballs and trackball-mice are interesting in that there is no universally-recognized form factor; the spheres come in a variety of sizes and locations, as seen in the Kensington, Logitech and Microsoft versions above.

One of the more unusual we've seen is this "one-finger mouse," produced by an unknown manufacturer and re-branded/sold under various marques like BXT, Fellowes and Rolee Polee.

How to actually use the thing is not discernible through photographs; for that you'll have to see the video below.