Who would have though you would be able to make money from your mobile phone network, well with giffgaff you can! They call it payback and you can earn it in various ways.
The most obvious one is recommending them, either by getting people to sign up using your referral link or by handing out sim cards (You can request sims from giffgaff to hand out) You get 500 points for this and that converts simply to £5. If people don't use your link or have one if your simcards then you can still get payback if they add your giffgaff username to the 'who referred you' section when they register their new sim card. You only get 200 points for this (£2).
The next way of earning payback is a bit different, they say to help out giffgaff members in their forum. How you actually earn and how much is all a bit of a mystery as there have been several posts on their forums and nobody actually knows! All I know is that you get paid to use their forum!
Giffgaff email you a monthly statement of your points and you can also check your balance online and they pay out twice a year (June and December). What else is good is they let you choose how you would like to be paid. You can choose cash via PayPal (only eligible if you've earned more than 1000 points), Airtime credit or you can donate it to charity.
Between June and November 2013 I earned a massive £55 for my December payback! I think that's amazing as I only pay £10 per month for my giffgaff goody bag. My statement for the next payback is currently at £30 (for December and January!) I wouldn't say I am overly active on their forum - I signed in in 2012 and have only got 277 posts. I only really started using the forum when I realised you get paid for it (Would have been around May 2013). The forum is good though, lots of friendly people chatting about random things, as well as a really good help section if you need it.