Mac OS X
Mac OS X [msnbc.msn.com]
Mac OS X is acknowledged as the main commercial rival to the Windows monopoly, which is an impressive feat considering that it is not licensed out.
Every release has been named after a big cat. The latest, OS X Lion, changed the way people interact with desktops. The software contains a number of gesture-based features, and integrates Apple's iOS interface to make it familiar to iPhone, iPad and iPod users.
Lion is also a fraction of the price of Windows and is offered for download through the App Store.