German.net is a language-learning website with numerous exercises. Levels range from approximately A1 (beginner) to B2 (advanced). This could be a very helpful tool for those looking to brush up on German skills or trying to advance their language.
German.net has a number of grammatical exercises, including ones on verb tenses, case and declension, nouns, adjective endings, prepositions, and the passive voice. These exercises are neatly categorised and are easy to find on the website.
These textbook-like exercises require the user to fill in missing information, and the solutions give the correct answer. 
It also has “vocabulary builder,” which quizzes the user on words which grow progressively more difficult. 
Perhaps most unique to German.net, and also most useful, is the “reading” tool, which presents short texts and then quizzes the user on the content. 
German.net also features a verb conjugation tool, which is especially useful when reviewing for a test or trying to learn more irregular conjugations. 