Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
AmzScope - Find profitable products for sale on Amazon in one click, relying on statistics instead of intuition
Savelist - Found products you want but not ready to buy?
AMZScout - AMZScout extension will help you make a decision about picking a niche based on the actual data, not intuition.
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.
Jungle Scout - Amazon product research made easy.