Skype is excellent! I believe it utilises a P2P setup and was made by the creator of Kazaa if I remember correctly.
Basically it's free to use when calling other Skype users any where in the world and the sound quality is amazing.
If you want to make voice calls to landlines or mobiles then you use a service called Skype Out where you put credit on your account and it works the same as a pay as you go phone. Calls to America are cheap (like 1.5p or 2.5p per minute).
You can use the service anywhere in the world where you connect to the internet. For example, I run it on my pocket PC. If I am at an airport in America I can connect to their wireless network, log on to Skype and make calls for free to other skype users who are logged on or call anywhere in the world as though I was just at home.
Another cool feature is Skype In which is a service thats (about 35 euros a year at a guess) and this gives you a phone number that land lines can call you on Skype. One good use is that you can choose numbers in a few countries... so you may want to have a US number for Americans to call and a UK number for British folk to call you on. Both numbers work and are active and anywhere in the world where you log on, people can contact you on those numbers if they are not using Skype. If you are not online then Skype will take a voice message for you.
It's a cool service and well worth it!!
Another last cool thing is that you can get Skype phones which you use as a regular cordless phone but it routes the calls over IP and you pay cheaper rates then your average BT/NTL line.
