Mathematics

Khan Academy--Math Exploration and Skill Practice (By Grade)

Khan Academy-- By Content Area

Numeracy Tasks

PBS Math - Lessons and real world applications from PBS 

YummyMath--Real World Math Activities

OpenMiddle--Challenging Math Problems Worth Solving

NRich--Enrichment skills in Mathematics from the University of Cambridge

YouCubed - Math based on grade level, topic, task

Wild Math - A collection of games, activities and stories encouraging mathematical creativity

Middle School Advanced Math Courses - Finance/Life Beyond School - Tasks aimed at MS/HS students dealing with personal math beyond school

Math Planet - Applied mathematics for advanced MS through HS 

Diffitme - Multi-subject, multi-language differentiation program for multiple subjects 

STEM Activities  - Simulations and activities for all grade levels in Math and Science (can be translated)