Are there any mathematics "problem websites" similar to Project Euler?

Question

Are there any mathematics websites similar to Projet Euler, a website which hosts math-heavy programming questions, many of which can be solved with a pen and paper?

I've become almost addicted to Project Euler's progress tracking system, and I also absolutely love their solutions forums. I love reading about other solutions—about how much quicker they are, how much more intuitive or elegant they are.

All Project Euler problems directly involve math. Are there any mathematics websites which track progress, and have solution forums, that are similar to Project Euler?

I liked Khan Academy, but I've completed its most advanced classes in school already, so it isn't much fun anymore.

Answer 1

The AwesomeMath website--www.awesomemath.com--will lead you to the journal Mathematical Reflections, each issue of which has one or two short articles and lots of problems divided into Junior, Senior, Undergraduate, and Olympiad problems. Solutions appear in later issues.

Answer 2

Also ArtofProblemSolving hosts international, national and regional math competition questions from all over the world. Every question links to a forum to discuss solutions. Your progress isn't tracked, however.

Answer 3

The site Brilliant.org has many questions and various gamified ways of tracking progress. They've changed their system and format a few times, but it is always a source of math-heavy problems.

One of its founders is a frequent contributor and answerer on Math.SE as well.