Tetris

Level advance:
Hold Down and power on the Game Boy. Keep holding Down, then press Start at the title screen to begin game play at ten levels ahead of the level that was selected. A small heart will appear with the level number.
Information in this section was contributed by John Thompson.

Expert mode:
Press Start to pause game play, then press Select. The preview window will not show the next piece when game play resumes.

Dancers:
Dancers doing a Russian dance will appear after completing a level 9 height 5 B game; or passing 100,000 and 200,000 points in a type A game.

Slow motion:
Hold Up and turn on the Game Boy. Keep Up held and press Start. Select any level and the game will be in slow motion.
Information in this section was contributed by Beegee4282.

In-game reset:
Press A + B + Start + Select during game play.
Information in this section was contributed by tomsmi.

Rocket animation:
Get 600,000 or more points in a Type A game to see an animation of a rocketship taking off.