O2 are giving away 2 free sim cards on their simplicity PAYG tariff. (This is not a contract, you dont have to use the sim)
This is a great tariff which gives you:
Unlimited Texts for only £5.
Or you could get Unlimited Texts and Unlimited Internet for just £10.
Theres lots of other combinations.
Its free to request the sims from o2. Just fill out the form at the bottom of the page.
The best thing is mine came with £2 free credit on each sim. You can just use this credit if you wish and never actually use the sim. Its not a contract.
O2 also do top up suprises, o2 rewards, 10% back of topups etc.