Secret Prices is a comparison shopping tool. You can compare product prices across multiple vendors in 18 different categories including books, movies, music, electronics, and event tickets. The results are shown in a very readable tabular format and there's an inline shipping cost calculator, so that you can compare the true, as-shipped cost from each vendor. The Amazon Marketplace is well represented.
Interestingly enough, Marc told me that the lowest priced vendor doesn't always get the sale. There's a strong preference toward Amazon among his user base. That's good to hear, and there are undoubtedly many reasons for this. I'll take a guess at a few. First, people are loathe to create yet another online account. Second, they would like to deal with a known entity where possible. Third, they have probably had some positive experiences with Amazon in the past -- an easy shopping and buying experience, on-time shipping, and responsive customer service. Competing on price is almost too easy; doing a good job in these other areas requires vision, patience and a continued investment.
Nice work, Marc!
Update 1: This post was edited to correct the name of the developer. His name is Mark, not John (sorry, Mark). Thanks to Nasser of Frucall for pointing this out.
Update 2: And he spells it Marc, too!