Unique or discount code plugin

Hi

I will be creating a site for a band. They would like to give away unique promo codes for their cd, which will allow the purchaser to visit the site, enter their unique code and download an audio track.

Is there a plugin for WordPress that does unique codes? If not, we could do it so that all cd buyers get the same code, so we have a page where the same code is entered before visiting the download page.

Thanks in advance,

Ian