oh fuck, i forgot the 3DS had BC with DS gameslike what
that really opens up a lot of cool shit
Super Mario 64 DS
The World Ends With You
Ghost Trick: Phantom Detective
Professor Layton and the Curious Village
Animal Crossing: Wild World
Castlevania: Dawn of Sorrow
Phantom Hourglass/Spirit Tracks
New Super Mario Bros.
Mario & Bros. Partners in Time/Bowser's Inside Story (but only if you've played Superstar Saga)
Seven mainline Pokémon games
Countless good ol' Mario stuff
It also has Fire Emblem: Shadow Dragon, which is a remake and port of the first Fire Emblem game (the one where Marth is the protagonist).