I made an open source quiz/flashcard application for learning German vocabulary. You can see it here on the Play Store, or download it from F-Droid.
The word list is decent, but could use improvement. You can get a read-only version of it from this Google Sheet. If you want to change one or two words, it is probably easiest to email me those changes. If you want to do more, great! The app could use more useful sections and groupings of words, either using the existing word-set or adding more of your own. You may want to email me so I know what you are doing, just to coordinate if more than one person is working on it at once.
Other sources for the app:
The source code for the app is on GitHub.
The word list was created by Neil at http://germanvocab.com and is based on Jörg Tiedemann, 2012, Parallel Data, Tools and Interfaces in OPUS. In Proceedings of the 8th International Conference on Language Resources and Evaluation (LREC 2012).
The photograph of the Berlin TV Tower is licensed CC-BY-NC from Michael, user juseranotherhuman, on Flickr.