Grammar & Exercises
https://www.usingenglish.com/esl/students/ – grammar, exercises, tests
https://www.englisch-hilfen.de/ – grammar, exercises, tests
https://learnenglishteens.britishcouncil.org – authentic English texts, audio, and videos with corresponding tasks
Dictionaries & Encyclopedias
https://www.dict.cc/ – online dictionary (several languages) in cooperation with the TU Chemnitz
http://capl.washjeff.edu/index.php – picture dictionary
https://www.britannica.com/ – Encyclopedia Britannica is a digital encylopedia especially for British culture
https://www.thesaurus.com/ – synonym finder