Skype over the net - free.
i googled "how to call usa cheaply" where a listing says I can call the usa for 1p. But I am nervous about using such a service just in case its a scam.
There are numbers you can call in the UK which connect you on for a couple of pence a minute, I used them a while back and they were pretty decent.
voipcheap are free too, free to uk landlines too if you stick on a few quid every 3 months.