I am intermediate Java/Python/PHP developer, but I really would like to help build something for Ubuntu for free. How can I do that?
There is a very useful information in this regard on the Ubuntu App Showdown page:
The App Showdown should be closed already, but the information there is still useful. Basically, the quickest way to start is Quickly and Python.
The best part about it , there are so many ways to start . I will try an list some of them . So you can select the best to suite you.
How can you Contribute to Ubuntu , Ubuntu wiki to introduce the Value of contribution.
How to Get Involved , listing Various Groups and Parts of Ubuntu to Contribute with.
As a Application Developer , developing Apps through Quickly
Ubuntu Development Beginnings , one of the best way . As an opinion , try to begin from here
Also it was already addressed here Ask Ubuntu site as following
Further basic read on How to Contribute to Open Source