Refer-a-friend schemes, or 'refer-and-earn sites' as they are also known, can be great ways to boost your income online.
Plenty of companies and services have refer-and-earn schemes, largely because it's a cheap marketing tool - you do the work for them and it's cheaper than advertising.
You're probably already a subscriber to services that offer refer-a-friend (RAF) incentives and, even if not, there are lots that cost you nothing to join.
How much cash you can make depends on two main factors - which RAF sites you subscribe to and how many friends you have. We can't help you with the latter, but here's some of the best refer-and-earn sites available to UK customers.
TopCashback - £15
TopCashback is a portal that rewards members with money back when they buy goods and services from major brands via their site.
TopCashback members can get £15 when they refer a friend - and the referral gets a £5 giftcard.
Amount you can earn: £15 per referral.
Key terms: For the Tell-a-Friend bonus to be applied to your account, your friend must authenticate their TopCashback account and reach a minimum cashback threshold of £10 cashback.
Quidco - £10
Quidco is very similar to Top Cashback in that you can earn refunds on shopping for hundreds of items when you use their portal.
Key term: Your friends need to earn £5 cashback before you're credited with your £10.
GiffGaff - £5
GiffGaff is a budget mobile phone network that relies heavily on its customers to both offer each other support and spread the word. They offer £5 'payback' for every time you refer a friend and your friend gets £5 credit. You can spread the word via email, social network or by sending a SIM to your friend's address.
Amount you can earn: £5 per referral.
Key terms: Payback can be used as credit, converted to cash or used as a charity donation. The cash option is only available if you've earned 1,000 payback points or more. £5 is 500 points, so you need two referrals to convert to cash.
Sky TV - £50
Sky can be pretty pricy but you can claw a bit of money back with the refer-and-earn offer. You each get £50 on a prepaid Mastercard. If you're thinking of joining Sky and know someone who already subscribes, there's the option to ask a friend to introduce you. You can introduce as many friends as you like. To make it easier, you can introduce up to five friends on each invitation.
Amount you can earn: £50
Key terms: Friend must live at different UK address to you. £50 will be sent to both parties when new customer's first subscription fee is paid.
Nationwide - £100
Nationwide has 15million customers and wants them to spread the word about its services. Current account members can refer a friend to open a current account and, as of June 2017, savings and mortgage members can also make use of it. If you're already a customer, Nationwide's is one of the best refer and earn scheme's in the UK.
Amount you can earn: £100
Key terms: Refer a maximum of five friends per tax year. Your ‘friend’ must do a full switch of their main current account (at least two direct debits) into FlexAccount, FlexPlus or FlexDirect within 90 days of the recommendation (the switch must close their old account).
OVO Energy - £25 (gift card)
Energy supplier OVO give you and your friend a £25 gift card to spend at Argos, M&S or Amazon.
Key terms: Direct switches only apply - so a switch using a comparison website will not count.
Swagbucks - 10% of their earnings
Swagbucks rewards members for carrying out online activities such as watching videos, filling in surveys and even gaming. You get paid in Swagbucks, which can then be converted to gift cards. You can earn 10 per cent of your friend's earnings for life.
It does not mean your friend will earn any less.
Amount you can earn: Unlimited.
Key terms: No maximum limit but some categories do not count towards your percentage.
Slice The Pie - 10% of their earnings
SliceThePie lets users earn money from writing reviews on songs, adverts, accessories and fashion items before they are released.
Amount you can earn: Unlimited.
Key term: You earn 10 per cent of your friend's earnings for a year.
Aviva - £25 (gift card)
Aviva offers pretty much all types of insurance. If you recommend a friend who buys any eligible policy, you'll both get a £25 Amazon gift card.
Amount you can earn: £25
Key terms: The offer is only available on new home, car or van policies where the annual premium to be paid by the new customer is over £100 and they have not been a customer with Aviva for 12 months prior to their purchase.
Utility Warehouse - £20 (gift card)
Utility Warehouse's refer-and-earn scheme will give you a £20 give voucher to use at Amazon, John Lewis M&S or Sainsbury's for every customer you refer.
Amount you can earn: £20.
Key terms: Your friend will need to call 0333 777 3 212 to give your membership number.
Unshackled - £20
You can earn £20 credit for every friend you refer to Unshackled.
Unshackled is a service that sells unlocked phones on finance - there is no limit to the amount of friends you can refer.
Amount you can earn: £20
Key terms: Your referrals must be new customers to Unshackled.
We couldn't really go without mentioning our own refer-a-friend scheme. Platinum members can earn £10 for every friend they introduce to Profit Accumulator and there's a £25 bonus for every five referrals.
Amount you can earn: £10
Key terms: New sign-ups must use your referral code and become Platinum members for you to qualify.