Jungle Scout - Amazon product research made easy.
Skuuudle - Discover, match and monitor your eCommerce competitors.
Inventory Spy - Spy how many products remain on any Amazon Product’s offer listing page.
Price2Spy - Price2Spy is a price monitoring, pricing analytics, and repricing tool, meant to help eCommerce professionals keep an eye on their competitors or retailers.
AMZScout - AMZScout extension will help you make a decision about picking a niche based on the actual data, not intuition.
BuiltWith - Find out the technology behind websites