There are various programs available allowing VOIP (Voice Over IP) - this allows you to have a telephone-like conversation using the internet, by sending the sound from your PC's microphone to another person and playing it out using their PC speakers. However, many of them do not offer strong encryption, and even if they promise this, if the source code isn't available how do you know the vendor hasn't been persuaded by MI5 to put in a weakness allowing them to eavesdrop on your conversations? Crypto Speak allows you to set up an encrypted connecton and because all the source code is freely available you can be sure that there are no deliberate weaknesses or back doors.