You can make international calls on Straight Talk with a calling card using an access number and they are nearly free. I use one called "Lovers Card" (hehe) that I get from firstphonecard.com- about $18 buys me about 25 hrs of calling to Canada, for example- well less than a dollar an hour. Europe and S America are only a little more expensive.

A trick I used when I was on Verizon was to list my local calling card access number as one of my five "Friends and Family" numbers. Thus, any time I made a call using that number I was not charged minutes. I called Canada and France frequently and paid only calling card rates. Plus if I thought I might go over my monthly minute allocation I made domestic calls as well with the card, effectively transforming my limited plan into an unlimited minutes plan with only trivial calling fees.