Google just announced an extension to free domestic calls using Gmail and the Google Voice web app. I'm currently calling every phone in America. So yeah, I'm pretty excited about the news.
The free domestic calls are for the United States and Canada only. Sorry Mexico. North American users can continue to make free calls from Gmail or their Google Voice account using the web app. (But not Mexico.) If you have Google Voice tied to your phone, you'll still have minutes deducted from your plan. Of course, if you have a mobile phone you probably have more minutes that you know what to do with. But still, it's a nice gesture. [Google]