Excerpt from Wikipedia:
Skype (pronounced /ˈskaɪp/) is a software application that allows users to make voice calls over the Internet. Calls to other users within the Skype service are free, while calls to both traditional landline telephones and mobile phones can be made for a fee using a debit-based user account system. Skype has also become popular for its additional features which include instant messaging, file transfer, and video conferencing. The network is operated by Skype Limited, which has its headquarters in Luxembourg. Most of the development team and 44% of the overall employees of Skype are situated in the offices of Tallinn and Tartu, Estonia.
Skype Limited was recently purchased for 8.5 billion USD by Microsoft who announced that they planned to continue Skype development.