Paribus - Find out for free if online stores owe you a refund!
Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
Lolli - Earn bitcoin when you shop online
Fetchee - Fetchee is a free price tracking wish list app that works with all online stores worldwide.
ShadowBid - iOS App - A simple iOS app to automate saving money on Amazon
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.