Gmail Calling offers unlimited free calls to and from the U.S. and Canada.

Gmail, Google very popular free e-mail service, has been offering free Internet based calls to and from the U.S.and Canada regardless if they are made to a computer or to an actual phone with Gmail Calling. But Gmail Calling works best with a free Google Voice account, which assigns you a phone number that links to all your other lines – work, home or mobile. It acts like a personal switchboard operator, allowing you to decide which phone will ring when someone dials your Google number. Without Google Voice, you can only place outgoing calls through Gmail.
Gmail is hardly the first Internet phone service. Skype has offered VoIP calling to computers since 2003 and telephones since 2004. Skype's presence is greater with nearly any computer in the world able to call any phone or other computer. Gmail calling now works only for U.S.-based Gmail accounts and charges for calls to international numbers cost from 2 cents a minute to landlines in much of Europe and Asia, to 99 cents a minute to Cuba. But Gmail calling with Google voice is versatile, because it can handle calls to your other phones. For the moment, it's also cheaper for certain types of calls. On Skype, computer-to-computer calls are free. But calling a phone costs 2.3 cents a minute to the U.S. and many other countries, or 2.99 a month for unlimited calls to the U.S. and Canada. On Gmail, all outgoing calls to U.S. and Canadian numbers and all incoming calls are free, through at least the end of the year according to the Wall Street Journal.

For more, see WSJ.com