AliPrice - AliExpress Price Tracker app lets you track the price history of any item from many online retailers like AliExpress, Daraz, and Amazon, etc.
Groupon - Each day Groupon offers a deal through local vendors in your city.
idealo - Idealo is a free to use mobile application that helps you to find products, compare price and make an informed decision.
Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
Slickdeals.net - Slickdeals: The Best Deals, Coupons & Discounts on everything
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.