Cool! I think a cool idea for a related service would be crowdsourced local price comparison. Take pictures of items in store with an app, which uploads to your servers to do OCR/mturk and get the UPC. In exchange, users get coupons. Then when they want to buy an item, they go to your website to find the store selling it for the cheapest.

It would add accountability to the claims by big retail chains that they "match any price."

Great! Is there any call limit?

Up to 2000 calls/minute. We will hopefully increase this soon.

What are your data sources?

user contributions, purchased databases, paid moderation and web crawlers.

I happened to have a can of soft drink in my had so I punched the UPC in and it was not found (unsurprisingly, it's a small northern soft drink company in the UK) so I hit Add Product and then it asked for a signup.

I think you'd get better buy in if you could add the product first then sign-up or not have to signup at all but put those entries into some kind of moderation system.

wow, thanks a lot for your thorough advice :) tbh I really hate putting road blocks in front of people who would like to contribute to the database, however we were the victim of a huge spam attack with completely random ip rotations. I will eventually remove the mandatory sign up once we have a strong moderation system in place, this shouldn't be too far from now. Thanks again, I really appreciate your comment :)

if people are rotating IPs to dodge your blockers, maybe you could shadow-ban them instead? that way they don't know that the IP is bad.

That's true; the mandatory sing-up is a temporary measure, I'm sure there are a lot of ways to fight spammers including the method you mentioned. Thank you for your suggestion.