If you're looking for free sim cards then the good news is that T-Mobile, O2 and Orange all let you get hold of one (or two) for free! All you have to do is supply them with a few details and in a few days your sim cards will be sent to you. T-Mobile: Sign up today and get 2 free sim cards
O2: O2 will give you up to 4 free SIMs, each with 500 free UK texts a month.
Vodafone do have a free sim offer, but this is for Pay Monthly contracts only. On PAYG you pay £2 for the card, but get up to £2 free credit though.
With all cards do make sure you read the terms and conditions associated with the contracts and usage. Most offers are also restricted to one per household and are available whilst stocks last.