Australian VoIP player Freshtel, has been quietly making some moves. I just spotted their Mac Beta, but was more impressed with their calling rates.
Doing some quick math, I quickly realized that for 1.2 cents per minute in USA currency, one can be making international phone calls. Only drawback is you can't add a USA DID (yet). With their easy to use configuration guides you can certainly configure any SIP device or softclient like CounterPath's EyeBeam or Bria as well, as use their own softphone.
My prediction: Watch for more foreign carriers and SIP based companies to enter the USA market with advanced services and better International pricing.