From today 3 UK will reward customers with cashback for referrals.

Join 3’s Free Agent team and get paid for promoting our free SIMs online. You get £5 cash when each friend first tops up their SIM by a minimum of £10, and they get an extra £2 credit. There’s no limit to the amount of money you can make.

see also: http://freeagent.three.co.uk/