Memrise is a tool giving you access to hundreds of thousands of courses on any subject you want to learn about, even if you just want to brush up on what you already know. No matter what you want to study, this app probably has it, with, for example, more than 180 languages, including Toki Pona (an artificial language) and Esperanto (which is nearly extinct).
Among the topics included in Memrise, besides the languages, are the arts, sciences, mathematics, the natural world, history, and geography. All the classes have been made by professionals in each field and improved by users who've studied the subjects, ensuring that the classes are of good quality and adapted to your needs. When you begin a lesson, you can continue your progress from the main menu, which displays the percentage you've completed and what you still have to finish. This makes it easy to manage the time you spend on various courses if you do several at a time.
On the other hand, for users who want to focus on passing a certain test, Memrise also provides training for the SAT, GMAT, TESOL, or even the TOEFL certificate. The way it works is the same for everyone: you learn the most important concepts in each field with small cards and apply your new knowledge in tests. Gradually, Memrise will help you learn new things, or help you strengthen your knowledge on a particular issue or details you didn't know before on any subject or language.