Over the past few years Google has continued to enhance the service and today announced it was free to people in the United States. Here's a quote from the Google Voice Blog:
A little over a year ago, we released an early preview of Google Voice, our web-based platform for managing your communications. We introduced one number to ring all your phones, voicemail that works like email, free calls and text messages to the U.S. and Canada, low-priced international calls and more—the only catch was you had to request and receive an invite to try it out. Today, after lots of testing and tweaking, we’re excited to open up Google Voice to the public, no invitation required.
Google claims there are currently over one million active Voice users. After today it should become a lot more popular. If you have not had a chance to try it out check out this one minute and 32 second video from Google.
It is worth trying out.