One of the most maddeningly frustrating but hugely addictive games I've ever played. I used to have it for the Spectrum, and I poured hours into it. I even have a dim recollection of finishing it (I did used to finish games back in those days). The Speccy version was excellently done, but I'm choosing the arcade version for its smoothness and colour.
The aim of the game is simply to bump into your adversaries and force them off the edge of the playing area. They're trying to do the same to you, and there are more of them. You can get items that increase your bumping power and let you jump over gaps... aaaand that's about it! Simple stuff, but there's something uniquely compelling and tense about being in the midst of several enemies, trying to manoeuvre yourself into a controlling position.
Oh, and if you're taking too long on a screen the squares start getting destroyed one by one.
Motos is a wonderful little game, one that sprung from a simple initial idea and carried it out without overdoing things. It has that special purity that the best arcade games share.