Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
PriceSpy - PriceSpy helps you save money when shopping online.
Fetchee - Fetchee is a free price tracking wish list app that works with all online stores worldwide.
Savelist - Found products you want but not ready to buy?
Price Snoop - When viewing a product on Amazon, Price Snoop will show you just how much it costs on the other...
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.