Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
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.
Fetchee - Fetchee is a free price tracking wish list app that works with all online stores worldwide.
Honey - Honey is a browser extension that automatically finds and applies coupon codes at checkout with a single click.
Pricepin - Pin products from webshops & get notified when prices drop