Google had introduced the Google Voice feature in Gmail back in August 2010. This new feature allowed users to make calls to anyone in US for free using Google Voice and had competitive rates for other countries.
The service was very popular and 1 million calls were made in the first 24 hours. Initially, Google had said that the service would remain free till December 2010, but they extended it to remain free through 2011.
Today, Google has added another feature to the service called "Click to Call" where they automatically link the phone number included in an email message.
When a user clicks on the link, Google will pull up the dialpad and automatically populate the phone number into it. In addition to that, Google has also added a quick dial icon next to the phone numbers in your contact list to allow you to quickly make a call.
Skype also has a similar feature available through their toolbars, however, in case of Skype, you can quickly make calls to numbers listed on any webpage, including Gmail.
I use the Gmail Phone call service a lot to place calls within the United States, this new subtle addition will definitely make things easier for me.