These are resources that have been associated with the broader national Math Circle network.
NY Times Numberplay: https://www.nytimes.com/column/numberplay
NCTM Illuminations: https://illuminations.nctm.org/
Three-Act Tasks:
101 Questions: http://www.101qs.com
Illustrative Mathematics: https://www.illustrativemathematics.org
"Exemplary"-Designated NSF Funded Math Curriculum: https://www.nsf.gov/pubs/2002/nsf02084/chap1_4.htm
Shell Center: https://www.mathshell.com/
Mathematics Assessment Project:
Robert Kaplinsky Lessons: https://robertkaplinsky.com/lessons/
EmergentMath:
Annenberg Learner: http://learner.org/resources/lessonplanbrowse.html?d[]=MATH
YouCubed: youcubed.org
NRICH Math: https://nrich.maths.org/frontpage
Balanced Assessment: https://hgse.balancedassessment.org
Would you rather: http://wyrmath.com
Desmos Polygraphs (Guess Who): https://teacher.desmos.com/polygraph
Open Middle: http://www.openmiddle.com
Mathalicious: http://www.mathalicious.com
Estimation 180: http://www.estimation180.com
NCTM Problems of the Week: https://www.nctm.org/pows/
Fawn Nguyen: http://fawnnguyen.com/making-problem-solving-part-math-curriculum/
Number Talks: http://www.mathtalks.net/teachers.html
Which One Doesn’t Belong: http://wodb.ca
A World In Motion: https://www.sae.org/learn/education
Museum of Science: https://www.mos.org
PBS Design Squad: https://pbskids.org/designsquad/parentseducators/resources/
History of Math Archive: http://www-groups.dcs.st-and.ac.uk/~history/
Visual Patterns: http://www.visualpatterns.org
MathWorks Math Modeling Challenge Tasks: https://m3challenge.siam.org/practice-problems
Fermi Problems:
Buck Institute: http://www.bie.org/resources
Sample PBI Regional Initiative: Math https://my.pblworks.org/resource/video/math_and_molecules_matter