Nintendo mods don't get much cooler than this. It's a Super Mario Bros. cartridge from the original NES, and it just so happens to have an entire NES crammed inside of it. Yes, an entire console.
It has the ports for controllers where the old connectors used to be, connections to run it to your TV at the top, and a space for the cartridge to pop into on the flat side where the label is. It's a pretty amazing feat of moddery, one that pretty much one-ups every other retro console mod I've seen. I could see these selling relatively well to hobbyists if they were put up for sale.