I use the service from FreeCall.com (one the VoipAround brand names -- voipbuster is another). I have a client in Switzerland and also routinely call Germany (from the US) and it works very well for me. No inbound US DID's (phone numbers) at this point, but may come later.

Another solution for the traveller is VoiceStick.com -- they put there software on USB stick that us usuful if using somebody else's computer. They have US DID numbers.