With a media player, some skill with a soldering iron and a little bit of imagination a modder has stuck a NES into a NES Cartridge.
As you can see from the video, that's a fully working NES emulator stuff in to a NES cartridge, with an actual controller built in as well.
The mod is work of Airz, from the forums of veteran modder Ben Heck. Airz said of the project:
"My nesP is basically a noname china brand portable media player, with the advantage of being able to play nes roms, and a nes controller hacked to the controls. First off the buttons on the original media player just sucked, they were so small and tight that my fingers could not easily push them. So I got the idea of modding a nes controller to it ... it worked perfect, it was like it was made to fit."