We do not mention every dial around international calling service but Spokn really stuck out when Josep sent me a link to it. The service kind of reminds me of Skype but cheaper. The international calling rates are pretty good and they even let you get your own direct number in 8 different countries for a dollar a month!
The service is compatible with SIP phones along with Windows desktop clients and is getting a Mac client soon. As I said the service kind of stands out from the crowd with striking similarities to Skype with lower prices.