A few weeks ago we came across earthcaller.com offering free VoIP calls to US and Canadian numbers.
Now the developers ( Jaduka.com) have launched a new web activated service called Duka Dial offering the same free calls to the US and Canada. It is mainly targeted at US and Canadian residents as you require an incomming North American number to use the service, but there are ways and means to get hold of a free US number and forward this number to your current phone where ever you are in the world.
All you need to do is register (for free), enter the number you wish to call, enter your number and click the "Start" button. This works for both landline and mobile numbers.