PriceCute - Keep up with your competition's pricing on Amazon
AliPrice - AliExpress Price Tracker app lets you track the price history of any item from many online retailers like AliExpress, Daraz, and Amazon, etc.
Mac Buyer's Guide - Recommendation on when to buy Apple products
idealo - Idealo is a free to use mobile application that helps you to find products, compare price and make an informed decision.
Pricy - Amazon price watch
Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.