Learn German Vocab (LGV) is an English-German learning-aide. I am using it myself to learn German, because all the other apps that I have tried so far have not quite met my needs (and/or proved too expensive to expand). I wanted an app with long lists - the sort I had at school, only better.
With LGV:
+ Lists can be sorted alphabetically;
+ Favourite (or tricky) words can be starred;
+ Known words can be crossed-out;
+ Word of the day, from your list of tricky words;
+ Nouns are colour-coded by gender, and include plural endings;
+ Lists can be filtered by favourite categories;
+ Tutor mode steps you through learning the categories;
+ Customize which words are tested: choose categories, grammar type, noun gender, verb tense, etc;
+ A virtual sheet of paper is included, so that you can test your knowledge the old-fashioned way;
+ Advanced users can edit the vocab list and add their own words and categories (via the Shared Folder in iTunes);
+ No In-App Purchases, ever! Vocab updates are free!