Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
Slice Watch - Get an email when an item you've added drops in price 💸
Fetchee - Fetchee is a free price tracking wish list app that works with all online stores worldwide.
Lolli - Earn bitcoin when you shop online
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.
Paribus - Find out for free if online stores owe you a refund!