How to make free calls on your iPhone with Google Voice

1. Google Voice is still invite only so if you don't have an account you can request one here: https://services.google.com/fb/forms/googlevoiceinvite/ Or ask a friend who already has an account to invite you.
2. Log in to your Google Voice account (http://voice.google.com) and under Settings on the Phones tab make your iPhone the one that Google Voice forwards calls to.
3. To set up your contacts. You can import from a number of sources or manually add them. To make it easier to navigate on the iPhone I suggest you only add the 10-15 people you call the most.
4. Now log in to http://voice.google.com/m/ in Safari on your iPhone. Navigate to the Contacts and everyone you set up in the previous step should show up. Click one of the names and Google Voice should call your iPhone and then connect the call to your contact all for free without using any of your minutes on your calling plan.