From Cpedia Mobile:
Google Voice Dialer for Android will routes your call through the internet and will not use your minutes. Google Voice Dialer for Android will intercept your outgoing call, uses Google voice to call you back and connect your call for free. You can use Google Voice Dialer for Android with Gizmo5 or sipdroid. It can also use it when not in US.
What's new in this version:
version 0.6.1 fixed the Google voice login issue.