SIP (Session Initiation Protocol) is probably the most widely used standardized protocol in use for VoIP. Countless SIP based VoIP providers are available offering a wide range of services, prices, and quality.
While Google Voice is attractive to many at the moment because it's free for US calls, Google Voice does not offer 911 service whereas many SIP based VoIP providers do. Some VoIP providers offer better international calling rates than Google Voice. Google Voice offers next to no support.
Google Voice is the relative new kid in the VoIP arena and uses a non-standard (XMPP) protocol. The OBi is the only hardware based solution supporting Google Voice.