Most of the handset manufactures have their own proprietary OS. They allows application developers to develop applications using Java ME or BREW.
In case of smartphones, it uses Symbian OS, Windows Mobile or Linux. All of these supports OS level programming (native programs), and they may also support BREW or Java Runtime.
In Symbian, Series 60 interface is most popular, mostly used by Nokia, but there are others like SE, Siemens, Samsung which also have Symbian handsets.
NOTE: Series 40, Series 60, Series 80, Series 90 are user interface and not OS. Nokia Series 40 interface runs on nokia's proprietary OS, while others run on Symbian. In case of Sony-Ericsson, the Pxxx series of handsets run on Symbian OS with UIQ interface.