Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
Honey - Honey is a browser extension that automatically finds and applies coupon codes at checkout with a single click.
Fetchee - Fetchee is a free price tracking wish list app that works with all online stores worldwide.
Invisible Hand - InvisibleHand gets you the lowest price on shopping and flights.
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.
RetailMenot - The RetailMeNot mobile app allows you to find deals on the go for both online shopping and in store shopping.