PriceCute - Keep up with your competition's pricing on Amazon
LoveSales - Find and save products you love, get alerted when they go on sale.
Yellowbag - Save your favorite products and buy when the price drops
TrackIf - Alerts you if the price of your favorite products drop
Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
Giftwhale - Giftwhale allows you to create personalised wish lists online.