I know OScommerce and Zencard allow for downloads. It's worth taking a look at them, or doing a google for IPN download script. The first result i got was http://www.deliveryscript.com/
If you are looking for easy and free script then Simple IPN is one of good script for digital downloads. Link : http://www.minisitegallery.com/blog/simple-ipn.html
The are much better and more streamlined alternatives for selling paid downloads. Click my signature link and check out the digital ecommerce shopping cart since it is probably exactly what you are looking for.