The latest beta version of “Skype”:http://www.skype.com/ – the popular phone software which lets you conduct free calls between PCs anywhere on the Internet – now lets you dial any regular telephone in the world too. It’s not for free but doesn’t cost much (land lines in most industrialised countries seem to cost Eur 0.012/$.014 a minute, though calling most mobiles costs much more).
The “Free World Dialup”:http://www.freeworldialup.com/ network (accessible via “various software packages”:http://www.freeworldialup.com/support/software_downloads) used to offer free calls to ‘normal’ phones as well as free PC to PC calls – it may still for all I know – but it is rather tricky to set up, while Skype is known for its straightforward installation. I haven’t tried either myself recently so caveat downloader.