Dig Dug is another of my favorites.  You are Dig Dug, the underground miner.  You have a jet powered shovel which you use to dig tunnels underground by moving up, down, left or right through the dirt.  Your enemies are the fire breathing, dragon-like Fygar and the balloon-like Pooka.  Your goal on each level is to destroy all of the Fygar and Pooka by either pumping them up until they explode or walking under a rock so that it falls on them and crushes them (be careful, the rocks can cruch you too!).  There are four different levels (colors) of soil on each screen and the deeper you are when you destroy an enemy, the more points you get.  You can also pick up vegetables (which appear in the middle of the screen throughout the game) for bonus points.  This game is very common and can be easily found for cheap.

Hint: It seems that a vegetable will appear shortly after you have made 2 rocks fall on each level.