If you have a smartphone, checkout MobileVoip couple with its voipcheap plan. 10 euros every 90 days gives you 300 minutes a week of free calls to the US, Canada, and many other countries. Calls above that are cheap, I don't recall the fee.

The client is VOIP, call through, and callback. If you have free incoming calls on your plan, you don't burn minutes. I do VOIP over 3g and sometimes it works great. Other times, it is not so good.