Megamania is another really cool game from Activision.  It is based on the concept of Space Invaders: you are a space ship at the bottom of the screen moving left and right, shooting up, trying to hit enemy "aliens" that fly across the screen while you avoid being hit by them or their missiles.  Megamania does have a few twists that make it both more fun to play and more interesting.  The enemy aliens in this game are in the form of such objects as Tires, Bow Ties, Steam Irons, Dice, Hamburgers and equally random enemies.  The movement of the enemies varies from one level to the next, so the game doesn't get stale as quickly as many similar games.  Also, you have an energy bar at the bottom of the screen serving as a time limit to destroy all of the enemies on each level.  If your energy bar runs out, you lose a ship.  You can also steer your missiles.  In my opinion, Activision scored a hit with Megamania.  This game was very popular back in the 80's, so there are a lot of copies around.  You should be able to pick one up at a good price.