Jungle Scout - Amazon product research made easy.
Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
AMZScout - AMZScout extension will help you make a decision about picking a niche based on the actual data, not intuition.
Fetchee - Fetchee is a free price tracking wish list app that works with all online stores worldwide.
Inventory Spy - Spy how many products remain on any Amazon Product’s offer listing page.
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.