Michael Robertson, AKA "the Disruptor" wasn't happy with only doing unto Skype before Skype doeth to him so he launched OpenSky. Now he's taking aim at GoogleVoice with Guava-Google Unauthorized Android Voice Access.
Basically, this is possible due to a peering relationship between Gizmo and GoogleVoice that dates back to the GrandCentral days.
What does Guava do? Simply put, you use your Gizmo credentials, log into the app, and start calling as it makes calls out via GoogleVoice over WiFi.