Contributing end-user documentation
Concrete5 is a PHP open source Content Management System. During my internship at Ingram Micro Netherlands I worked on the new website of the local office. I used Concrete5 for that website so the employees there could easily update the website on their own.
For the employees that were going to use it I wrote an extensive user documentation about the Administrative panel of the CMS. When it was finished in English, and later in Dutch, I shared it with the Concrete5 community.
I've put the original source files on GitHub to share it with everyone.