A cool if somewhat rare theme in video games, the classic example of which being Tron - a game in which the player attempted to trap the other players inside their trail, causing them to humiliatingly crash and die. Less noteable would be adaptations of old paper and pencil games in which players connected dots in an attempt to reach the other side or create enclosed shapes (usually squares or rectangles). Also noteable are 2-player Tetris, and Dr. Mario. Finally, any game, regardless of theme, with ricocheting weapons and friendly fire make it possible to Humiliate the Other Player through Application of Geometery. I fondly remember a tank game for the old Atari where you could take out the other player with a sweet-ass triple bank shot.

Thanks to all of the noders who reminded me that the tank game was named Combat.