I get the feeling I've seen this before somewhere, and the creator, dany32412 on Instructables, himself says it was inspired by another source. That's still a functioning Nintendo Entertainment System within a modified NES cartridge, which is a great novelty mod. The difference here is the original gamepads are supported.
NES in a Cartridge [Instructables]
DISCUSSION
Huh, shows you how much of the NES was wasted space if he was able to shove all the essential parts into just a NES cart.
Not as cool as the Ninten-toaster, but still cool.