Many people will think you are pulling their leg if you tell them that you can quite literally get cash for free. But you can - and millions of savvy savers are raking in the pounds every time they go shopping.  Lynn Nicholson was a doubter, too, until she looked at her bank statements over the past 12 months and was shocked to see that she missed out on ?718 in cashback from cashback websites.  Ms Nicholson, 31, does around 80pc of her shopping online. She missed out on ?232 cashback when she moved house in June and purchased a new home insurance policy from aviva, and signed up for new contracts with BT and Sky. Other retailers with missed cashback opportunities include Ryanair, Gap, Dorothy Perkins and Oasis.  She said: "With some money-saving websites you find you may actually spend more money just to save a few pounds. But I can see from my bank statements how I could have earned cash back with normal spending habits." Ms Nicholson said she would be signing up for a Quidco membership after seeing her potential savings.  She is not alone. Cashback websites are hugely popular and boast an estimated 2.3 million consumers. The sites list product providers and retailers that pay commission when shoppers click on their links. In turn, the cashback website passes on the commission to the consumer. Some retailers pay a fixed sum, while others pay a percentage of your spend.  The concept behind these websites is simple: each will pay a cash reward when you visit one of their retail partners via the site. all you do is sign up and provide your bank details. Membership costs about ?5 a year - although you will not be charged if you do not earn any cashback - so you really have nothing to lose.  To use the sites is relatively simple - register with the site and then, every time you buy online, visit the retailer via your chosen cashback operator rather than directly. The retailer typically pays a commission into your account at your cashback site, which in turn sends your cut direct to your nominated bank or PayPal account.  Here we look at the five biggest cashback websites.  1 QUIDCO  www.quidco.co.uk  With more than 2,842 retailers signed up, this is the biggest and highest-paying cashback website. You can earn ?70 when you buy car insurance from aviva or ?62 when you sign up for dual fuel at Scottish Power. Unlike other cashback websites, you can also earn money in stores. Simply register your credit or debit card details and every time you make a transaction at one of the partnered retailers the money is sent directly to your Quidco account. For instance, you can earn 4.5pc cashback at NCP car parks and Cineworld, and 2pc at Debenhams and B &Q.  You can also search for shopping vouchers and discount codes that can be entered into the retailer's website, so you receive the discount as well as the cashback.  2 TOP CaSHBaCK  www.topcashback.co.uk  This website boasts it offers the best earnings as it does not charge an annual fee. With 2,550 partners, Topcashback is the second biggest cashback website. Shoppers can earn ?120 if they take out an iPhone contract from Vodafone, or you can earn ?65 if you sign up for a Virgin Media entertainment bundle. Other examples include 8pc on online purchases at House of Fraser and 5pc on down