Math is real:
Why word-problems are not the solution to math real-world math education:
Teach Now related:
Differentiated, adaptive Common Core-aligned problems (I only did explore math problems):
More adaptive learning by US Team Math Olympiads coach Po-Shen Loh:
Curated problem sets, organized by theme and grade:
I get interesting insightful problems from the Eton College entrance examinations:
All fora in the Art of Problem Solving. The books are great for differentiated instruction and to get new ideas.The community is really helpful. There's a wealth of resources in there, too long to list. Great for all math instructors.
Terry Tao's blog is an amazing resource. He is the foremost mathematician of our time and often shares his thoughts on math education and research. As a teacher, I think I should at least have an idea of what the current mathematical frontier is. Comments to individual posts are sometimes as good as the post itself.
Problem driven instruction:
Project Euler has been around for a while. It posts easy to understand problems (number theory mostly), which can initially be solved by hand and later using a programming language of choice. A good introduction to computer science and number theory, and something I would like to adapt to high school math one day. Maybe calculators could be replaced with computers + basic coding skills?
Desmos is a step in that direction,
but not quite there yet.
I also like MIT's http://mathlets.org/
And also interactive math:
Becoming a headteacher by leading a challenging school:
In case I missed anything, a list of curated resources (like r/random but occasionally useful), because it's refreshing to look at something other than math!