Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.
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?
PriceGrabber - Get the lowest prices on computers, electronics, furniture, cameras, clothes and more.
LoveSales - Find and save products you love, get alerted when they go on sale.