A Japanese startup named Beatrobo presents an interesting way to bring the game cartridge to the smartphone era-- the Pico Casette, a tiny cartridge one plugs into device headphone jacks to unlock games.
Unlike its larger, retro brethren the Pico Cassette does not actually store games. Instead it transmits an inaudible tone to authenticate games on a companion app. In addition it also connects with Beatrobo servers to allow users to play and save progress on multiple devices.
The technology behind the Pico Cassette is called PlugAir, and is already used to sell physical music and video content at Lawson HMV stores in Japan.