Now there are many OS available in the market. Some of them are now open source too.
Listing out some popular Mobile Operating Systems
* Android OS (Google Inc.)
* Bada (Samsung Electronics)
* BlackBerry OS (Research In Motion)
* iPhone OS / iOS (Apple)
* MeeGo OS (Nokia and Intel)
* Palm OS (Garnet OS)
* Windows Mobile (Windows Phone 7)
* webOS (Palm/HP)
* Symbian OS (Nokia)
and some others are
Aliyn, Firefox, Ubuntu touch, Sailfish, Maemo, Grid, etc.,
