The DeutschAkademie app offers over 22,000 vocabulary and grammar exercises

Learn German 6,000 Words categorizes words by 15 topics and 140 sub-topics

Der Die Das has an archive of 17,000 words to get you memorizing the German vocabulary

King of German Article has an endless gender quiz

AnkiDroid flash cards

Yapping Puppy offers German learning games

German Verb Trainer offers customizable tests