The Symbian platform is a free open-source software platform for mobile devices. From Symbian^3 onwards you can get any of the source, modify it, and contribute back the changes.
The source code is organised into a number of discrete packages, as described in the Symbian System Model. It comprises about 25 million lines of code (around 40 million if you include the test and tools code) in 200,000+ source files.
Here are a few links to help you get started: