Tetris DS (Nintendo)
This game has unused graphics.
Nintendo's two-screen rendition of Tetris, themed after classic NES games. It introduces plenty of new modes, too, like Catch and Puzzle.
An unused, possibly early playfield graphic.
A Super Mario Bros. flagpole and end-of-world castle against a black background. In Classic mode, Mario at the top of the screen never makes it to the end of a level, making this go unused.
Yoshi's Other Eyes
On the bottom screen of Puzzle mode (themed after Yoshi's Cookie), Yoshi is seen looking to the left. However, it turns out that the image itself has Yoshi looking to the right. The game puts a separate graphic over his eyes to make him look the other direction.
NES Game Leftovers
Having more detail is always a good thing.
Specifically: Rip the sprites and put them here.
Every level in Classic Mode has unused sprites from the original NES games themselves.