We came across this VoIP service today offering free PC to Phone calls to the USA and Canada. Although we have not tried the service ourselves there seem to be no strings attached. Their site offers a very simple interface and no account is required for the free calls and no hefty software to download.
It appears that this service has been going for a few months now and recently Earthcaller.com started offering calls to other destinations as well for pretty decent rates. For example calls to a UK mobile are only 11.9 US cents and calls to Australian mobiles only 10.3 US cents.
We have no idea what the quality is like, but if anybody has tried them please drop us a note or leave your comment below.