The MATtours sites of ISEM provide user-guided tours of mathematical ideas.
The ellipse site focuses only on ellipses. It was designed as a prototype for a grant proposal that was never funded. A visitor can choose a formal tour, a tour based on questions, or a modeling tour, switching among them or skipping around as curiosity dictates. Although the site hasn't been updated for a long time, its pages still get several hundred views a month.
A tour of discrete mathematics was developed under contract with SciMath Minnesota in the U.S. for preparing high school mathematics teachers to teach discrete mathematics. It allows users to jump around among theory and applications as desired. Unfortunately, the state-of-the-art programming of the time doesn't work on modern browsers, so you can probably see only the splash page and not the other windows that should open.
Currently under construction is a tour of all of mathematics from preschool through master's-level ideas (in the U.S.), with many topics not traditionally addressed in curricula as well. Each page contains an assertion and questions arising from that assertion. The questions link to assertions on other pages. Curiosity might take a visitor from elementary mathematics to very advanced mathematics and back. We hope to have the site ready for review in the summer of 2018. If you're interested in alpha or beta testing, please contact us: copes (at) edmath.org.