Fring is a social communication app that works with just about every social networking tool out there. They have just released their fring app for android which will allow you to make free VOIP calls to your contacts without using your cell phone minutes. The application integrates with your phones contact book, so while in your Fring app, simply find the person you want to contact and select call.
The only limitation is that the other person must also be using Fring for the free calling to work. But you can purchase credit to enable Fring to call any landline or mobile phone. Check out Fring.com for more info.