Google Voice is a phone service provided by the makers of Gmail. It is an extremely well priced and competitive phone service that has VoIP rates which are literally pennies. In fact, if you are in the USA then calls are completely free throughout North America. The service also offers a number of other perks, such as voice transcription, conference calls, SMS to email, voicemail and even voicemail message to email services.
Orpheum offers its iVoice (SIP trunking) and hPBX (Hosted/Virtual PBX) services exclusively to businesses in the USA and Canada. Both services start at $25 per month.