Wow, this is huge. Google has made it so you can make free local and long distance calls (to US and Canada), and cheap international calls from directly inside Gmail. If that wasn't cool enough, you can even receive phone calls right inside Google Talk if you are either away from your phone, or you simply don't have one.
You can now feasibly get phone service anywhere in the United States for absolutely nothing. You get a Google Voice number, and a Gmail account, and you magically have unlimited inbound and outbound calling -- free.
You don't need a physical phone to use Google Voice anymore, and that is big news. The technology behind this feature comes partially from the acquisition of Gizmo5 not so long ago.
In addition to the free calling from your computer, Google is also rolling out old school phone booths in airports and campuses. These magic booths will provide free calling -- but I think it's mainly going to be used to help get the word about Google Voice out there.
Have you tried the new feature yet? What do you think? I've personally given it a whirl, and was really impressed by the sound quality from both ends.