These are systems that give you money in return for you making a purchase or signing up for something. Sometimes you have to spend some money to earn something back, and sometimes you don’t.
The best and most reputable cashback website is TopCashBack. TopCashBack is a fairly new cashback site that has the best payouts, even compared to GreasyPalm. Their minimum cashback payment is £0.01, and they can even pay you by Paypal (as well as by Cheque and Bank Payment). Their friend referral bonuses are low however, 50p when each friend reaches £5 in their account. However, that’s because they give you large cashbacks.
The oldest cashback website is Greasy Palm, with probably the best range of retailers to sign up with. They have more retailers than pretty much any cashback site, although they do have the lowest cashback rates. If you refer your friends, they offer the best bonuses (£7 when each friend reaches £15 in their account) compared to other cashback sites though.