Totally agree with Moob - Skype is the best free software!

Call quality is better than normal phone lines... If you haven't tried it then do!

I also subscribed for their 'Skype Out' service which allows me to call almost anywhere in the world for around 1p a minute (landlines - mobiles/cellphones are a bit more), though the quality isn't quite as good as Skype to Skype calls (because it's using traditional phone services I guess).