Slickdeals.net - Slickdeals: The Best Deals, Coupons & Discounts on everything
Camelcamelcamel - Easily monitor Amazon product prices and get alerts when they drop.
Groupon - Each day Groupon offers a deal through local vendors in your city.
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.
Keepa - Price history charts and the option to be alerted on price drops for all Amazon sites.