My Father linked me to this website when i was having a long-distance relationship, it's part of Martin's Money Tips:

The blue ones require an account, the green ones are simple dial-and-go ones, and red ones are safe to ignore being from BT or OneTel etc. I thought I had a deal paying 4p/min to call South Africa until I found one for 2p/min, so thats slashing my bills in half.