GrooVeIP now has a free lite version
Google Play (market): https://play.google.com/store/apps/details?id=com.snrblabs.grooveip
Amazon Appstore: http://www.amazon.com/snrb-Labs-GrooVe-IP-Lite/dp/B007HA7A18
I have the app, and it is really useful. They finally came out with a lite version that is free. It has ads supporting it, plus a couple features are disabled:
- 3G calling is removed
- Native integration is removed (you have to use the app to dial; using the built-in dialer won't give you the option to call via GrooVe IP)
If you want the full version, it goes on sale from time to time for $2 from amazon app store.