One is Ebates. I have heard a lot of people say that they believe Ebates is a scam. I can tell you, from personal experience, that Ebates is for real and I regularly receive checks from them. Now they have added in-store shopping to their site! Just look for your store before you go, select your deal, link the card you're going to use, and you can earn cash back in a physical store also!
Here is the link to sign up for Ebates:
Another addition is Wikibuy. Along the same lines as Honey, which we discussed last week, it automatically applies coupon codes to your order. Here is the link for Wikibuy:
While some of these sites/extensions may only give you a few percentage points back...remember that every penny counts!