Easy addition: NES controller

[ArduinoFan0]

It is literally just a shift register with buttons attached to it, so you could easily implement this.
I believe the pins labelled N/A Data are for the NES Zapper.
Also I found these images on google (though couldn't find one of the button to byte map)

[RenaKunisaki]

SNES controller is exactly the same, you just shift 16 times. (The last 4 bits aren't used by controllers but are used by eg mice)