Cambridge Write & Improve (for students to practise their writing and get automatic feedback): https://writeandimprove.com/
Cambridge Speak & Improve (for students to practise their speaking and get automatic feedback): https://speakandimprove.com/
Cambridge English YouTube channel: https://www.youtube.com/channel/UCnXqRankqlC47qzXcYMazKA
Level test for students: https://www.cambridgeenglish.org/test-your-english/
https://textinspector.com/#section--try-analyse-text (250 words max. with the free version)
English Profile (to check level of vocabulary): https://www.englishprofile.org/
English Profile (to check level of grammar): https://www.englishprofile.org/english-grammar-profile
ALTE: https://www.alte.org/
British Council EAQUALS core inventory: https://www.teachingenglish.org.uk/sites/teacheng/files/pub-british-council-eaquals-core-inventoryv2.pdf
CEFR level descriptions: https://www.coe.int/en/web/common-european-framework-reference-languages
CEFR (2001): https://rm.coe.int/1680459f97
CEFR Companion volume: https://rm.coe.int/common-european-framework-of-reference-for-languages-learning-teaching/16809ea0d4