Future Learn : I’ve used this quite a lot, it is great for adults or children, I like the format, if you have completed part of a course it marks it in another colour, it is easy to get what you want from a course. You can skip bits, or return to them. A good range of courses too… all sorts. Well worth a look.
Alison : This looks similar to FutureLearn but with more emphasis on certification. It may have more courses suited to ‘school’ levels.
University of Cambridge Free taster courses and some free courses, worth a look for specialised interest.
BBC on-line adult courses though of course these needn’t be for adults. Good for brushing up on basic skills and more.
See also : Educational sites and Apps