PriceCute - Keep up with your competition's pricing on Amazon
Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
Mac Buyer's Guide - Recommendation on when to buy Apple products
Fetchee - Fetchee is a free price tracking wish list app that works with all online stores worldwide.
Pricy - Amazon price watch
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.