There are many cash back sites on the internet, my three favorite are upromise, ebates, and bondrewards.
Upromise is designed specifically for college students and their families. A student can sign up for it and give their family a link and the entire family could get cash back for the college student. However it gets better, there are upromise surveys which will give you one dollar every once in a while as well as a few non purchase click throughs which you can get a bit of extra money. There is also a link to erewards.com which is surveys that can get you up to an extra $50 a year.
You can link upromise to a high yield savings through Sallie Mae and get an extra 10% of the money you have received from upromise in the the high yield saving. You need to set up automatic transfers of $25 a month to the high yield saving to get the extra money though. The interest rate on the Sallie Mae high yield savings account is about average for high yield saving.
Ebates is similar to upromise, however ebates.com is a little more high tech. Whenever you earn more than $5.01 they send it to your paypal account, no work on your part. That makes ebates superior to upromise. I love how easy it is to get the money. However, you may not get the money unless you have over $5.01, for over a year I had $3 sitting there not earning interest or anything because I had not gotten the amount I needed. However $5.01 is not a lot of money and ebates is the only cash back site that gives cash back for purchases on ebay so for that alone, makes it worth the two to five minutes to set it up. Also ebates allows for referrals. For every person you refer and they make once purchase you get $5 as do they. However that $5 is not included in the $5.01 that you need to earn to get the money.
BondRewards is the most unusual of the cash back sites. Bondrewards will only give you the cash back once you have reached 50 bond dollars, but 50 bond dollars do not equal $50. You can cash in 50 bond dollars for $25 or a $50 E bond. However to buy a $50 E Bond only costs $25 but the bond will not be worth $50 for another twenty years. Therefore when comparing bondrewards to other cash back sites, divided the percentage back by two to be able to get a equal comparison.
Depending on what website you are buying from will determine which cash back site is best for you at that time. A lot of the time, the cash back will be identical however each cash back site above has websites that the others do not, that I buy from and given that it does not cost me anything to sign up, I use all three. I find the website with the best deal and then go to all three sites and see which site gives me the best cash back. It only costs me five to ten minutes per purchase and it normally saves me about three percent of the purchase.