Back in 2005 I was fortunate to get a Grand Central invite via Mike Qaissaunee. Grand Central was a web-based phone service company that was purchased by Google in 2007 for $95 million and relaunched as Google Voice in March 2009.
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:
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.