I've been working on a project that needed the ability to convert a UPC to an Amazon Standard Identification Number, or ASIN. There don't seem to be any simple services to do this -- I needed an Amazon Associates account just to make the request to look it up.
I have created UPCtoASIN.com. It's a simple web service for converting a 12-digit UPC (or 13-digit EAN) to an ASIN number.
For example, the ASIN for the UPC
B000I5XOVY. This can be looked up using this URL: http://upctoasin.com/012569828490