AmzScope - Find profitable products for sale on Amazon in one click, relying on statistics instead of intuition
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.
Jungle Scout - Amazon product research made easy.
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.