• Unlimited conversation time with user using internet.
• Incredible voice and sound quality, even on a modem 56Kb
• A nice GUI which look the same on all plattform (Windows | Mac OS X | Linux | Pocket PC), with a search engine to locate Your friends.
• Firewall, NAT and proxy friendly, no worries about network configuration. (It use the port 80 like a browser)
• Completelly free version. it does not contain spyware or advertising. You can even call someone on a regular phone (without internet) by paying yearly fees (Skype out).
• End-to-end encryption: every data leaving Your Pc are encrypted (SSL 128bits)
• A great community up to 20 Millions users (Forums - Community)

Skype has recently see the start of some intersting plugin for it, especially for adding Video to it

• vSkype www.vskype.com adds video and sharing to your Skype experience. Getting started is simple. Just connect a camera, and invite up to 200 buddies to a video call. It does not exist right now but may appear soon.
• Dialcom's Spontania www.video4skype.com video plug-in is designed to be integrated with any 3rd party Internet-based communication solution. It is right now available for skype (windows only :-( ) and it work perfectly.

In order to enjoy video with a computer, I recommend You to buy a GOOD webcam: (sorry to say that I did not have any luck with 2 basic logitech webcam at 60 euro). Buy at least: 640x480 pixels at 60fps (120euro), nothing else! Philips webcam are known to wook good in low lights conditions. it's a more expensive combo but You will never regret it!