Check this out, http://www.sipgate.co.uk/user/tour.php

Sipgate is a free Internet telephone service provider that allows ya to turn your laptop into a phone. Gives ya a new 020 number and works through your broadband line. Its free to make calls to other users and the quality is pretty good. You can even make calls to LAN lines and mobiles and the charges are cheap. Pass the word on and get everybody on it!