Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
AliPrice – AliExpress Price Tracker - AliPrice – AliExpress Price Tracker app lets you track the price history of any item from many online retailers like AliExpress, Daraz, and Amazon, etc.
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.
idealo - Idealo is a free to use mobile application that helps you to find products, compare price and make an informed decision.
Fetchee - Fetchee is a free price tracking wish list app that works with all online stores worldwide.
PriceSpy - PriceSpy helps you save money when shopping online.