What is German Grammar Spy (Der Die Das)?

German Grammar Spy (Der Die Das) is a study aid for German learners to help them master the most tedious and difficult part of German grammar: the articles. It does this by teaching the ~50 rules/signals for gender (e.g. "nouns ending in -ung are usually die"), focusing on the 5000 most frequent words, and employing SRS to schedule revisions.