Star Wars (Sega Master System)
This game has a hidden level select.
As expected, based on the famous 1977 movie of the same name. No Scorpion Darth Vader here, unfortunately.
At the title screen, press Right, Up, Down, Right, Up, Down, Right, Up, Down, Right, Right, Right. The music will change to confirm correct entry. Start the game and intentionally get a game over. The level select will be on the continue screen. Press Left and Right to toggle the level to go to, and choose Yes to go there.
Levels 1B, 20, and 30 are all named "Unused" and simply go back to the copyright screen when selected.