Skype

The service allows users to communicate with peers by voice, videos, and instant messaging over the internet phone calls may be placed to recipients on the traditional telephone networks. Calls to other users within the Skype service are free of charge, while calls to land line telephones and mobile phones are charged via a debit-card  based user account system. Skype has also become popular for its additional features, including file transfer and videoconferencing.
Skype has 663 million registered users as of September 2011. The network is operated by Microsoft, which has its Skype division headquarters in Luxembourg.

For Download Skype