https://www.illustrativemathematics.org/

Illustrative Mathematics is a discerning community of educators dedicated to the coherent learning of mathematics. They collaborate to share carefully vetted resources for teachers and teacher leaders to give children an understanding of mathematics and skill in using it.

http://illuminations.nctm.org/

Illuminations is a project designed by the National Council of Teachers of Mathematics and is the primary contributor of resources for teaching and learning mathematics for grades pre-K—12. Illuminations works to serve you by increasing access to quality standards-based resources for teaching and learning mathematics, including interactive tools for students and instructional support for teachers.

http://www.teacherspayteachers.com/

This is a site full of lessons, assignments, and other resources created by teachers. It offers ideas for all subjects and grade levels, and while many products have a fee in order to use, there are also free resources.

http://www.nctm.org/resources/content.aspx?id=9284

National Council of Teachers of Mathematics lessons plans and activities for High School Mathematics.

http://nlvm.usu.edu/en/nav/vlibrary.html

The National Library of Virtual Manipulatives is a resource from which teachers may freely draw to enrich their mathematics classrooms. The materials are also of importance for the mathematical training of both in-service and pre-service teachers.