Sites with interesting and varied Math Problems and Puzzles to challenge students

​http://mathriddles.williams.edu

http://mathschallenge.net

http://gregtangmath.com/materials

http://mathforum.org/dr.math/

http://figurethis.nctm.org

https://brilliant.org/

http://www.homeschoolmath.net/online/favorite_puzzles.php

https://plus.maths.org/content/

http://www.openmiddle.com/

http://www.cut-the-knot.org/

http://mathpickle.com/

http://www.matholympiad.info

http://www.nytimes.com/column/numberplay

http://www.archimedes-lab.org/wp/

http://www.radicalmath.org/