Google has added a new feature to GMail – you’ll need to download a new plug-in (and be in the US or Canada initially) and you can then just place calls from your GMail to any number. Calls within the US and Canada will be free for the rest of the year with other countries such as the U.K., France, Germany, China, or Japan for as little as $0.02 per minute.
“We’re rolling out this feature to U.S. based Gmail users over the next few days, so you’ll be ready to get started once “Call Phones” shows up in your chat list (you will need to install the voice and video plug-in if you haven’t already). If you’re using Google Apps for your school or business, then you won’t see it quite yet. We’re working on making this available more broadly – so stay tuned!”
If you have a Google Voice phone number, calls made from Gmail will display this number as the outbound caller ID. And if you decide to, you can receive calls made to this number right inside Gmail.
Source: GMail Blog