Yars' Revenge

Hit a swirl in mid-air with the cannon. Then find the 'Ghost of Yar' (a black, vertical line that appears behind the explosion graphics) and position yourself on the lower third of it. If done right, when the explosion ends, the game will stop and the letters HSWWSH (for Howard Scott Warshaw) will appear.

This was the fist Atari 2600 game to display programming code on the screen. The Neutral Zone is comprised of actual game code, which is laid out vertically, and counter-scrolling over itself (along with some X-Y and random color processing).