The majority of my high-quality mathematical exposition is done with Math Advance. I strongly recommend you head over to MAST in order to get better at math contests, and MAC to learn about our contest.
Therefore, what you'll find here is mostly recommendations, reflections, and personal compendiums.
I won't list Math Advance resources here because they can all be found at the Math Advance website.
A list of mock contests that I've written for and mock contests that I personally recommend.
Evan Chen's olympiad program, which recently has had an RPG component added to it! I've taken many pages out of his book for MAST.