National Association of Math Circles http://www.mathcircles.org/
SIGMAA on Math Circles for Students and Teachers http://circles.sfprep.org/dept-computer/SIGMAA-on-Circles/
Math Teachers’ Circle Network http://www.mathteacherscircle.org/
James Tanton's Thinking Mathematics!
Aspiring Teachers http://www.aspiringteachers.org/
