We've always loved coupon sites like RetailMeNot for finding online discounts, but the Coupons at Checkout extension takes it a step further by integrating its coupon data into the checkout process.
The extension is available for Firefox, Chrome, Safari and Internet Explorer. Once it's installed, you'll find the section for coupons or promo codes when you get to the checkout stage at over 100,000 online retailers. In my tests, I had to sometimes click on the text field, click outside of it, and then click on it again to get the list to appear. Nonetheless, several results were provided in order of usefulness.
Coupons at Checkout is a free extension and runs hidden in the background, so you'll only see it at checkout.
Coupons at Checkout (free) [Coupon Follow]