Extending Drupal is where you find all the action. If you have a module that works then upload it up or send it in. Sellers should take a look at the vendors section.
Writing documentation and instructions. It's not as hard as you might think. Just tell your story on why something did not work and how you got to work.
Building websites from scratch using Drupal as a Framework. Creating installation profiles and distributions of Drupal.
Code added the DDN will be tested and approved or marked as untested. If you see a code snippet then look for the DDN approval. If it is not approved then join in and see if you can get it to work.