Google has extended their offer to make free voice calls within USA and Canada from their Gmail service throughout 2012.
The offer was to end this year and now they have extended it. Google said:
As the holiday season approaches, we’re happy to announce that we’ve extended free domestic calls within the US and Canada for 2012.
Google allows making international calls too but those are charged for. This service is also available in some international markets where all calls are charged for.