These sites offer a percentage of your purchase back to you - approximately 2 - 10% of your purchase price back. It is always worth checking to see if a company is on these before you complete your purchase. This is a list I've compiled recently, but new sites may be added. If you come across any that you have good faith in, please leave a comment to let me know and I'll add it to the list.
Try these sites:
UPromise - save for college while shopping
I recommend you check each site to see which offers you the best cash back on your purchase. However, if you are pressed for time, you can try Ev'reward. They will tell you which site offers the most back, but it may not be up to date.
You must shop through the site's link in order to get the rebate. If you are using Mr. Rebates, search for the store you will purchase from - and click on that store through the Mr. Rebates site. You will get the credit that way.