Free Cashback Websites
The best cashback websites are completely free to join and use. You should avoid the ones that charge any kind of membership fee, because this fee is unnecessary. Cashback websites make money when you shop through them, so it doesn't make sense for a cashback site to charge you to join.
The Best Cashback Sites
Shop Money - Shop Money offer a very good amount of cashback for thousands of online retailers and are very reliable.
Rpoints - Rpoints is a very well known and trusted cashback website that has its own highest cashback promise, which means they can't be beaten.
Making Money From Using Cashback Websites
You don't have to purchase goods to earn cashback. You can also perform various other online transactions. You can earn cashback for doing your car insurance online (often you get around £30 cashback for doing this), you can earn cashback for joining an online bookmaker and placing a bet (usually around £15 cashback), you can earn cashback for opening a bank account online (usually around £20 cashback), and you can even earn cashback for joining free websites.
From my own personal online shopping in the past 2 years I have earned £800 simply from using a cashback website, and I don't even spend that much money shopping online, but it does soon add up.