... and then found an NES controller buried in a box.
Thanks to pinouts.ru, I rigged up an NES controller adapter using the Arduino. Don't really have anything to use with it, but I suppose it's nice to have around.
The eight leds light up according to the buttons. NES controller plugged in according to this pinout:
1 o o 5
2 o o 6
3 o o 7
7. Data Out