features of Skype

1. peer to peer, no server required, therefore, no operation cost like cellular communications
2. overlay, not relying on existing Internet protocols beyond Layer 4, resilient to malfunctions of Internet services
3. scalable, using supernodes among every 500 nodes, adaptive to constant configuration changes
4. high quality codec for crystal voice
5. strong penetration to NAT and firewalls
6. free for now and maybe forever for on-net calls
7. implementation of packet voice
8. consumes reasonable bandwidth, consumes significant CPU
9 . teleconference, voice communications for 5 parties at most
10. millions people online all the times, number keep rising.
11. provide presence like messengers, great feature compared to old phones

Related link about Skype, Gizmo, and Google Talk
eBay acquired Skype