Keygrip:
Feel free to merge into the locked sticky. Tried sending a PM a while ago, your inbox appeared full.
As of 9th September, you'll need to pay for an ESTA. See
BBC News - US to charge £9 for Esta compulsory travel entry form for details.
As it lasts for two years, why not apply now - even if you did recently - and you'll not need to pay.
As an aside, any reason why the UK shouldn't start charging US visitors for visa / visa waivers ?