The Dreamcast VMU was really ahead of its time.
Primarily a memory card for Sega's final home console, it had its own built-in LCD screen and controls, which allowed it to host very basic mini-games based on the titles you played on the main console.
While we're sure that this isn't the first instance of this happening, but modder GGEAR_TARO has turned the VMU into a tiny Genesis / Mega Drive console.
Footage is shown of the modded VMU running Streets of Rage and Space Harrier II, and, small controls aside, we're pretty certain we need one of these in our lives right now.
The Dreamcast VMU has recently been upgraded by Dreamware Enterprises in the form of the VMU2, which offers more storage, a better screen and much more besides.