I've seen this question here and there before. People who want to write something, but don't know what, or basically just want to learn.

The good thing for you, however, is open source :) Just read through some source and try to understand it. If you wish, try to improve it or just add new features (just for fun (and send patches to the author of course ;))

A good thing would also be to look at what some piece of software does, and then immitate it. Write your own app that does the same. Compare it to the other source and learn from it.

Or just join a project over at SourceForge. There is always help needed.