Super Smash Bros.
The 3DS version of Super Smash Bros. will let you listen to the game's soundtrack while the handheld is in sleep mode.
Along with being able to listen to the game's diverse soundtrack in the game's sound test mode, which will contain both new, old, and remixed music, you will be able to listen to character voices, and set up special playlists. You will also be able to close the system, keep listening to music, and using the 3DS' should buttons to track forward and backward song to song. It essentially turns your 3DS into a dedicated Nintendo music MP3 player.
Game director Masuhiro Sakurai breaks down the new music options in more detail on the Miiverse here. Sakurai also confirms that select music tracks will be coming to the official Super Smash Bros. website, soon.
For more on Super Smash Bros. and its many reveals, head here.