Skype
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 phonesinstant messaging, file transfer, and video conferencing. can be made for a fee using a debit-based user account system. Skype has also become popular for its additional features which include
Skype was developed by Estonian developers Ahti Heinla, Priit Kasesalu and Jaan Tallinn, who had also originally developed the peer-to-peer file sharing software Kazaa.[1]
Skype Limited, the company that operates Skype, was founded in 2003 by Niklas Zennström and Janus Friis with headquarters in Luxembourg.
eBay acquired Skype Limited in September 2005 and in fall of 2009 sold a majority stake to an investor group.

[Update: on 9 August 2010, the company filed for an IPO]