LearningApps.org

LearningApps is a collection of interactive learning modules. It is a Swiss non-profit organization (so if you're able to, and find it helpful, you could consider donating to keep the site alive 😊) 

They warn users "The exercises themselves do not represent complete learning units. They are intended for practicing and deepening skills (e.g. in foreign language learning) and are not suitable for explaining complex concepts. Exercises published by users vary in quality and need to be checked before being used in class." So don't hesitate to ask me for explanations, and if you're doubting an exercise, screenshot it and send it to me, we'll see together. 

What other unknown apps do you know ? Comment below to share with others.