I started https://regressionbuddy.com last year. The idea is it's a site with math practice problems. It's more for if you already know the math and don't want to forget it. Each question has a corresponding entry in the appendix so you're never left hanging. New problems and appendix sections get added each post. A ballpark guess is it will take about a hundred posts, which works out to four years, to cover everything.

So far though there has been little interest in the site. Given how much work it will be to finish I'm afraid of wasting my time on something that is of little use to anyone.

I'm trying to figure out if I should let it go, to be perfectly honest. I don't want to but I don't think the payoff will be what I imagined.