This project will show you how to create a real USB gamepad using a Wii Classic Controller. The Instructable (full step by step instructions) is here: http://www.instructables.com/id/USB-Wii-Classic-Controller/
A demonstration video http://www.youtube.com/watch?v=-vp_u5k_xGc
The code will be based on V-USB, and the circuit will be based on a implementation V-USB example circuitry called USnooBie.