Sign up to O2's new 'Unlimited' P&G tariff online and you can get up to four free sim cards.
If you top up with £10 you will then get unlimited free texts to all other O2 numbers.
Top up with £15 and you get free calls & texts to O2 numbers.
Calls to any other numbers are 25p/min for the first 3 mins/day then 5p/min for the rest of the day.
If you already have an O2 Pay & Go sim, you can switch to O2 Unlimited by texting LINK to 21300.
Sign up through quidco for 15p per sim.
On top of that, you can win VIP tickets to the O2 wireless festival when you top up any amount.
I hope this hasn't been posted already, but couldn't find it anywhere. I thought it's a pretty good deal!