There are tutorials out there for using an NES
controller with an Arduino, but using the more complicated N64 controller and its analog joystick has a definite appeal. If you have an Arduino around and don't want to buy an Adaptoid, this instructable will make it possible to play emulated games in Project 64 with your Ardunio and an N64 controller.
Is this hard? / Will it damage my stuff?
This will not modify your controller in any way, and the wiring is extremely simple, so if you do everything carefully there should be no risk to your controller, and you can unhook it at any time to use with a N64 console.
What you will need:
3 pieces of wire - free (I hope)