Google Inc. is an American public corporation specializing in Internet search. It also generates profits from advertising bought on its similarly free-to-user e-mail, online mapping, office productivity, social networking and video-sharing services. Advert-free versions are available via paid subscription. Google has more recently developed an open source web browser and a mobile phone operating system. Its headquarters, often referred to as the Googleplex, is located in Mountain View, California. As of March 31, 2009 the company had 19,786 full-time employees. It runs thousands of servers across the world, processing millions of search requests each day and about one petabyte of user-generated data each hour.