Koules is a fun, original, and extremely addictive little arcade game for X. It is writen by Jan Hubicka, of AALib and XaoS fame.
You play a spaceship (cleverly disguised as a colored circle) who has to destroy other, smaller spaceships that try to bump into you. When any ship collides with another, there is a super-elastic collision that sends them both flying in the opposite direction, with varying speeds. You are generally stronger than the enemy spaceships, but they outnumber you. The more enemy ships you destroy without dying, the stronger your ship becomes. Spaceships are destroyed when they are forced into one of the four walls. Enemy spaceships sometimes change form when they hit a wall instead of being destroyed.
Higher levels bring new challenges. For example, there are stationary mines that destroy anything that hits them, black holes that suck ships toward them, giant super-powerful enemies that knock you halfway across the screen, and enemies that attach little bungee cords to you. This game gets hard, fast.