This site from Drexel University was one of the earliest comprehensive math sites. It has many resources for teachers and students. This particular page is for teachers, but there are student pages, "What's New" pages, and more.
Interesting tool bar-can download from CyberSmart, and you are guided to several sites they recommend. In some ways, I like to generate my own, but they undoubtedly have many I'd overlook.
Readings with free activities that are lexile-leveled. It is designed for grades K-6, but there are many readings with lexiles scores of 800-1020. Also, it provides lesson plans aligned with various children's books.
You can create a free account. Rec by Karen Ault