Study League combines a precise study timer with game-like elements. You earn points for study time, compete in leagues or real time revision matches, and can use your points to "buy" virtual players or unlock achievements. AS WE'RE CURRRENTLY IN BETA, there is no AI subject specific quiz mode, that is coming SOON
Of course its free. I (sdotdev) aim to put experience first, payment later. There is a MvpMODE for more features however you can do majority without and it's only a one-time payment