Discussion on: What's one resource most devs don't know that you use regularly?

I can't tell unless I could literally mind-read what most other devs think!

Personally, I read reddit (programming sub), quora, HN, FreeCodeCamp, medium, and dev.to, of course. All of them are quite popular so I'm assuming most other devs might be using them too. There are some other quiz, coding contest and pure fun sites too, and some of them are a little less known I believe:

1. open.kattis.com/
2. app.codility.com/programmers/chall...
3. hackerrank.com/dashboard
4. techgig.com/home
5. mathsisfun.com/
6. codechef.com/
7. pyschools.com/quiz/view_ranking
8. codingbat.com/
9. codewars.com/dashboard
10. mathblog.dk/programming-challenges/