[^http://www.symbian.com Symbian OS] is a fairly new operating system, aimed for use on mobile phones. Symbian OS is an open system in the sense that anyone can develop applications for it, without having to pay any license fees.
Symbian OS comes in several flavours, depending on the hardware it's supposed to run on. For instance, the Series 60 Platform is used for phones with number keypads, whereas the UIQ Platform is designed to support phones with a touch-screen.
SDK's and information on Symbian OS development can be found in the [http://www.symbian.com/developer/index.html Developer Section] on [http://www.symbian.com]