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