Kobun Dojo is a free game for mastering classical Japanese vocabulary and auxiliary verbs — the easiest points to earn on Japanese exams — while climbing through dojo belt ranks. Missed words are tracked so you can drill only your weak spots.
The content itself is classical Japanese (interface in English), making it a rewarding challenge for advanced learners of Japanese. It runs in your browser with nothing to install.
Answer the same question correctly twice and it graduates into a mastery card; the ranking counts how many cards you have collected. You can drill only the questions you got wrong, and every day you play adds a study stamp. No sign-up is required and your records stay on your own device. (The questions themselves are in Japanese.)