Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
PriceSpy - PriceSpy helps you save money when shopping online.
Paribus - Find out for free if online stores owe you a refund!
Savelist - Found products you want but not ready to buy?
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.
ShadowBid - iOS App - A simple iOS app to automate saving money on Amazon