I agree with neoufo51. Truphone is fantastic. It runs off the built-in sip client on the phone, no extra software to run... You just get a wizard application from them to help you provision the sip settings.
Truphone is on par with vonage. Good sound quality.
SIP is an open-protocol VOIP scheme (unlike skype which is proprietary). I think Vonage is the most well-known SIP provider, but there are probably hundreds of smaller ones.