In August Google launched the free phone service for US. and Canada which was powered by Google Voice. The "Call phone" button at the top of your Gmail chat list lets US Gmail users dial any phone number using their Google Voice numbers. This service was free till the end of 2010. However, Google has now extended the free calling through Gmail till 2011.
Note: Calling in Gmail is currently only available to U.S. based Gmail users.
To learn more, visit gmail.com/call.
Via [Gmail Blog]