A free, open-source, and unofficial Android app, Guava, gives any Android phone the ability to make and take calls over Gizmo5's VoIP service, connected through a Google Voice phone number. It works over Wi-Fi, 3G, or, for the daring, EDGE. You probably won't want to use Guava as your primary phone call manager, as the call quality varies with your connection and really works best over Wi-Fi. That said, if you're running up against your minute allotment, or find yourself in a basement-like spot with decent Wi-Fi but really bad cell coverage, Guava is a great little tool to have at your disposal.
- Carlos Leiva Burotto