A good version of the ever popular Space Invaders. Well designed, smooth graphics, with an effective sound accompaniment.
You start with three guns, and another gun is said to be available after 1,000 points. To verify that, though, more expert hands than mine would have to operate the controls, which are the Z and X keys for dodging right and left, and the space key for firing.
Both the current and the high score are shown.
Though the principle of this game is well established by now, this implementation is particularly good. Worth the money on that basis alone.