I am surprised when I opened my Gmail inbox today. What I see is a calling service service offered by Gmail!
This is what Google told us about
You can now place calls to landline and mobile phones from within Gmail. Just dial any phone number and you’ll be connected. Calls to the U.S. and Canada are free in 2010. International calls are billed at our insanely low rates.
You have to download a voice plug in for using this service
Google voice interface simple and sleek and it got lot of features
Google voice service is currently offered in US only. They have plan of expanding this to additional countries