Ultimately, all of them come from the same family. I don't know who was first, but the story I heard was it was invented by an Israeli family and the various members had a falling out and each started their own version--which is why the Aquabot, Blue Diamond and Dolphin are all basically the same design--two or three motors (depending on whether the treads are single or doubled), angling handle, center motor vacuums up junk and sucks the thing down so it sticks to the wall.

I figured out that since there aren't any sensors in these things, they must sense the change in current draw when they hit an obstacle and they can even analyze that change to determine whether to reverse or try to climb the wall. Kind of like when a power saw is pushed TOO hard and the lights dim. Since they were all family there must have been some agreement on patent sharing too.