Google officially released the ability to call people from within Gmail, a feature that existed in Google voice already. All you need to do is visit gmail.com/call and install the software if you havent already and a phone will appear under your name in your chat list. Mine has yet to appear because I think they are running a little behind so I cannot test it yet. All calls to US or Canada phones are free using this service and very low rates to other countries. Rates are as follows: More info at:http://www.google.com/chat/voice/compare.html
If you already have the feature in your Gmail please let us know how it performs. Full rates here.