I'm recommending the two websites from Singapore. I like them for their spirit of generosity and public service.
The following is the direct quote from the OpenlySolved website.
OpenlySolved is a digital library of living documents (living documents are documents that are continuouslybeing edited and refined) containing fully-worked solutions to problems found in commonly used mathematics textbooks in Singapore ...
Conventional textbooks typically provide only the final answers to the problems. While these answers are useful for the better students, who can use them for verifying that their own answer is correct, they are not helpful for the weaker students who may not even know how to begin solving some of the questions. These students need help learning mathematics on their own, and what is useful is the detailed solution process, not merely the final answer. This is what the solutions in OpenlySolved provide - the detailed steps involved in solving each problem - not merely the final answer - all at no cost. Register (for free) to access the solutions and to discuss the topics.
OpenlySolved is also a community of people that use textbooks. It is hoped that the members of this community will get together in the forum to discuss the solutions, highlight any errors in the textbook, and help one another learn. Although some initial solutions have been provided, it is hoped that the community will solve the remaining questions, and make the solutions available to all the other members ...
Lee Chu Keong, Founder,OpenlySolved
P.S. Who am I? I am a lecturer at the Nanyang Technological University. I am grateful for what I make through teaching, speaking and training. It is enough.
The following is the direct quote from the NotesAcademy website.
"NotesAcademy is an online platform for students to share their studying material, techniques and tricks with one another. Everything on the website is available to anyone completely free of charge"
"Our mission is to facilitate sharing, collaboration and cooperation amongst students, in order to allow students around the world to excel and realise their full potential."
The following nine subjects are available currently: