Lode Runner
What made Lode Runner the must-have game to have is that it was the first game with a built in level editor.  Before Lode Runner, once one finished all of the levels in a game, it was time to move on to another game.  To make custom levels, one needed to program their own game in Basic.<p>With Lode Runner, one could open up the level editor, easily make a custom Lode level, then playtest and share it with friends.