These are the sites I use most often, but I HIGHLY recommend checking cashbackmonitor.com before you do any online shopping, because it will show you what site will give you the most money back. They often trade places and run special promotions for certain categories, so I always check there first!
This is hands down the highest paying cash back extension I have found. If you want to get a high cash back offer, just browse whatever site you're interested in after installing the extension, then the following day (within 24 hours), you'll receive an email from them with your cash back offer! So easy!
Capital One Shopping Referral Page
I have had only good experiences with this site and use them a lot when I purchase on health and supplement sites like iHerb or Vitacost.
TopCashback.com: Highest Cash Back Guaranteed
This site scans all available coupon codes for you automatically when you install their extension and makes it easy to save on your online purchases that way!
Honey
Newer site and offering higher cash back amounts right now! I got 10% cash back on my iHerb order recently. Seems to have a lower minimum for withdrawal than other sites, too.