There are many resources to support math outreach programs:
Journal of Math Circles: https://digitalcommons.cwu.edu/mathcirclesjournal/
National Association of Math Circles (NAMC), http://mathcircles.org
NAMC Math Circles YouTube Channel, https://www.youtube.com/user/MathCircles
Circle in a Box (downloadable pdf freely available via NAMC)
Mathematical Association of America Special Interest Group of Math Circles for Students and Teachers (SIGMAA-MCST), http://sigmaa.maa.org/mcst/
Math Teachers' Circles Network, https://www.mathteacherscircle.org/
Julia Robinson Math Festivals, http://jrmf.org/
We are also working to create a list of resources developed by MathPOW! participants: