Asteroids. My dad introduced me to this one and it was a bit of a bonding experience playing against his high score on our PC. (He was much better at it then me). The gameplay is pretty simple. You control a little triangle/spaceship moving it around the screen to avoid asteroids while shooting them, and when you hit they split into smaller and smaller pieces. Eventually just disappearing for good. Of course on occasion flying saucer ships would show and start shooting at you as well. The game actually has you deal with the ships momentum as if you were actually in space and despite the simplistic graphics and sound it does a terrific job of making blowing up space rocks a satisfying experience.
If you have never played this game I implore you to go try it now. It shouldn't be too difficult to find in some form but I heartily recommend you stick to the original on PC or the arcades and don't bother with the remakes.