Skype is a P2P (peer-to-peer) software founded by the entrepreneurs Niklas Zennström and Janus Friis, also founders of the file sharing application Kazaa.It enables you to make free calls anywhere in the world.
Calls between Skype and traditional phone numbers
Subscription calling plans
Video calling and screen sharing
SMS text messaging
Wireless hotspot network access
Third party add-ons
See more details here.