said by Mike Wolf:Have you thought about Vonage?
IMO, Vonage is a poor value for most users. An exception is those who usually call more than 1000 min./mo. to the international destinations covered by Vonage World or Vonage World Premium.
For heavy domestic use and a Vonage-like experience, I'd recommend VOIPo or Phonepower.
For a person with some technical skill and light to moderate usage, try Callcentric, Anveo or VoIP.ms, as already suggested.
A heavy user might consider Alcazar Networks (domestic), or Voxbeam, CheapVoIPInc, or CallWithUs (international) as a supplementary outbound service.