Clozemaster is language learning through mass exposure to vocabulary in context. The goal is to fill in the missing word in a given sentence. The missing word is the most difficult word in the sentence according a frequency list for that language, and the sentences are from the awesome dataset at Tatoeba.
I started the site just under a year ago to answer the question "what should I do after finishing Duolingo?". Since then it's grown to support over 50 languages, mobile apps, and thousands of users. It's useful for learners at almost any level, from beginner to advanced, and makes a great complement to Duolingo, textbooks, classrooms, etc. to practice vocab in context.
You can play the Fluency Fast Track which gives you a sentence for each unique missing word in order of increasing difficulty, jump in to sentences grouped by frequency from the 100 most common words to the 10,000 most common, or just play random sentences. There's also "cloze-listening" - hear the sentence first, then see it and fill in the missing word.
Thought Hacker New might find Clozemaster interesting and hopefully useful! It's still very much a work in progress - I have a bunch more features planned and I'm working to improve it all the time. I'm also open to any feedback and happy to answer any questions!