These sites I have used throughout GCSE and A-levels that I found useful and resourceful.
Quizlet is a great site with an app too where you can make online flashcards and can store your revision there. Also you can access resources that other students have done. Having your notes digitally is great as you can revise anywhere and its easy to take with you, instead of having loads of folders to hold. With Quizlet you can organise your notes into folders which keeps everything organised.
Memrise is a language site and it's good for vocabulary revision. I used it for French GCSE and found it very helpful and greatly improved my grade. It also has an app so you can use it whenever you have some free time easily.
Physics and maths tutor
This site is my holy grail, I swear by it. Physics and maths tutor has loads of past paper questions for different subjects but I mainly used it for sciences and maths. It also has additional resources such as required practical notes.