Recently joined and thought I would let others know, is by far the cheapest network in the UK. You can top up normally or buy a 'goody bag' which lasts a month and gives you a set amount of call time (depending on the price of the goody bag), unlimited texts and unlimited mobile data.
You get £5 of free credit upon activation of your SIM and can easily transfer your old phone number across. Also, if you recommend anyone else to GiffGaff you get £5 'payback' which can be used as either credit or be received as a PayPal payment.
GiffGaff runs off the O2 network so anyone outside of the UK who is interested would have to check availability.
Anyone intersted can order SIMS through my link below.