Here is an annotated list of potentially useful resources, starting with my old blog, QED Insight [mathematics, science, education, and more]. I haven’t updated it since 2013, but it still has a significant number of visitors per year. At some point I will migrate the content over to this site, but in the mean time the content is still there if you would like to check it out.
The rest of the links to potentially useful resources are at the following pages:
Learning Mathematics and Science
Problem Solving and Contest Preparation
Other Interesting Mathematics Links