opularity. Skype gives you an open window to talk free to 340 million people worldwide. With Skype, and other applications of the like, you can talk for free with anyone in the world on condition that they are using the same service (Skype in this case) as well. So with that many subscribers, your chances of talking for free is much bigger. Most business contacts, friends and relatives do have a Skype presence.
Skype's call quality is also an asset. Its quality is not the best around, but reasonably good, as long as the conditions for a good VoIP communication are gathered. It has its own SIP and P2P standard, and a good number of servers worldwide to ensure good quality.
Also, its popularity has brought it to be adapted by many other services. For instance, it can be integrated with email clients like MS Outlook and Mozilla Thunderbird, with services like Jetnumbers virtual number service, etc. It is also the target application for most call-recording tools.