Gmail is having lots of great features. One of them is that you can make calls to any landline or mobile phone from Gmail itself. Here it is explained that how you can use Gmail to make phone calls

To make calls from Gmail, you must be signed in to chat and have the voice and video chat plug-in installed from the URL Now, just click the Call phone link at the top of your Chat roster to open up the Call window. Dial the number you wanna call and talk on it. Only problem is that this feature is available for USA and Canada users only.

Update : This Google Voice service is now available in India and many other countries. To know about the call rates of Google Voice for your country, visit the link given below.