These are some of my favourite websites because they are so helpful for many different reasons.
khanacademy.org - I have been using this website for about 5 years and it is magical. They have everything from science and math to art and humanities. They show you videos that aren’t ridiculously long and boring. Super helpful for everything.
codecademy.com - Great site if you want to learn how to code in depth or just get a small grasp on it. You learn by doing which is super helpful. They offer several different languages and it walks you though it while you are writing the code. They have different levels and and small projects like creating a solar system. Great for learning how to code especially for computer science and web design.
wolframalpha.com - I only use this website for last minute math assignments. You can enter your math problem and say what you need from it, like the maximum and it will give you the answer. This is great for those last minute assignments or for if you completely forget when its due and only have an hour, which is what happens to me.