Price2Spy - Price2Spy is a price monitoring, pricing analytics, and repricing tool, meant to help eCommerce professionals keep an eye on their competitors or retailers.
Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
Skuuudle - Discover, match and monitor your eCommerce competitors.
Fetchee - Fetchee is a free price tracking wish list app that works with all online stores worldwide.
Prisync - Prisync is a competitor price tracking and monitoring software for all sizes of e-commerce companies worldwide.
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.