Math

Illustrative Mathematics
Free, standards-aligned curriculum for K–12 with lesson plans and tasks.

CK-12 

Khan Academy for Teachers
Assignable content, videos, practice sets, and teacher dashboards.

Inside Mathematics
Rich tasks, Common Core resources, and classroom videos.

NRICH (University of Cambridge)
Problem-solving activities to stretch mathematical thinking.

YouCubed
Growth mindset tasks, PD videos, and math activities from Jo Boaler.

Open Middle
Challenging math problems with multiple solution paths.

Zearn Math (Free for individual teachers)
Digital math lessons aligned with Eureka/EngageNY.