Obihai Technology, Inc. via Amazon offers the OBi100 VoIP Telephone Adapter and Voice Service Bridge (pictured) for $39.99. With free shipping, it's tied with our July mention and the lowest total price we've seen. (It's also a current price low by $15.) This adapter supports Google Voice, SIP & OBiTALK VoIP services.
Obihai Technology, Inc. via Amazon also offers the
OBi110 Voice Service Bridge and VoIP Telephone Adapter for $49.99 with free shipping. That's $35 off and the lowest total price we could find.
It supports SIP for services like Callcentric, Sipgate, Vitelity, Voip.ms, and more.