Problems with solutions free online resources

At university problems were handed to us or we could attempt problems found in the course literature books. I haven’t done exercises like these in a while now and felt like going through my old literature but this time with the power of mnemonics, spaced repetition and lack of stress. Then I realized that there has to be mountain of these type of exercises with solutions online already if one can find them.

In this thread we discuss and gather free online resources of exercise problems with provided solutions. Feel free to post about any subject.

I think I’ll start with trying to get really good at Newtonian and Lagrangian mechanics. I’d feel really cool if I could get so good that problems eventually becomes more or less trivial as my mind jumps through all the steps to the solution.