1. Cashbaq -

2. Mr. Rebates

3. Ebates - Click here to sign up for Ebates and get cash back at over 1, ooo stores. You get $5 just for signing up.
4. Big Crumbs -

I check each site before deciding which one to use when I buy something. For example, if I wanted to buy something from OldNavy.com, I would see that right now I get 4% back from Mr. Rebates, 3% from Cashbaq, 2% from Ebates and 3.6% from Big Crumbs. With a different store, the percentages may be reversed and Ebates could be the highest. Each site runs monthly specials such as $10 off at entertainment.com for the 2009 Entertainment Book through Cashbaq and the daily double special at Ebates. You MUST log on to the cash-back site you want and click to the retailer through the site to get credit.
It doesn't make sense to not use these sites and get cash back on every purchase you make online. Sign up for all of them, bookmark them and use them every time you make an online purchase.