T-mobile are still giving away free sim cards and now they have a 5 day pass deal going on, which offers you unlimited anytime talktime to T-mobile phones for £5 over 5 days but my friends just used this for longer periods and used up around 1000 mins working out at just £0.005 a min. :)
Also if you top-up £10 in one go, they'll also provide you with free unlimited offpeak texts to all networks and also if you top-up £20 or more (combined for the month) they'll also give you 20% free credit to be used within either 90 days or 180 days (one of the 2 timescales but i can't remember which one).
This seems a great deal to me, since they already have probably the cheapest PAYG deals anyway, offering 12p a min at any time for any network or you can choose alternative deals, such as 5p a min to T-mobile calls but i think it would be better to take advantage of the 5 day pass deal and have 12p a min to any network.
Here's a breakdown of how to take advantage to the max with the free sim card:
1) Use the £5 for 5 days unlimited T-mobile calls (whenever you're ready).
2) Have unlimited offpeak texts to any network (£10 has to be topped up in one go)
3) and spend only 12p a min to any network and receive 20% back in credit (£4 for £20)
Hope that helps :)