I don't know if this is for all Gmail accounts but I have three and it works for all of them.
Google have integrated a dialer into Gmail, you can read more about it here - http://gmailblog.blogspot.com/2010/08/call-phones-from-gmail.html
Basically it lets you use Gmail to call telephones. Calls to the US and Canada are free. Apparently they always will be (funded through money gained from international calls).
There is currently no dialer for English (UK). However if you set your account language to English (US) then the dialer will appear and you can make free calls to the US and Canada.
EDIT: Apparently free until at least the end of the year.