CK-12

STEM and social studies resource with highly customizable content
Bottom Line: Making content accessible to kids with different learning styles, CK-12 could be the key to students understanding STEM topics.
Woot Math

Adaptive lessons, collaborative games, and analytics add up to an excellent math tool
Bottom Line: A top choice for teacher- or student-led lessons and real-time collaboration.
Khan Academy

Flexible learning environment offers goal-oriented personalized pathways
Bottom Line: A robust resource for targeted, individualized instruction in most standardized subject areas.
PhET Interactive Simulations

Massive collection of excellent (and free!) science and math sims
Bottom Line: Instantly add meaningful lab experiences to math and science classrooms for free? It's tough to beat that.
CueThink

Social approach to problem-solving promotes deep math learning
Bottom Line: An innovative, community-based platform that helps students plan, strategize, and collaborate.
Math Nation

Approachable video courses break down math, build up student skills
Bottom Line: This is a fantastic resource for students and teachers alike, with a straightforward, accessible approach to learning math and an admirable approach to differentiation.
CanFigureIt Geometry

Flex your deductive reasoning muscles with geometric proofs
Bottom Line: A really good way for math students to learn about proofs and explore them from a number of angles.
Desmos

Powerful graphing calculator deepens understanding with visualization
Bottom Line: Desmos combines interactivity, visualization, personalization, and collaboration to deepen students' understanding of geometry and graphing concepts.
Brilliant

Challenge top students with self-guided math, science, and CS courses
Bottom Line: This learning and practice site can be an exciting challenge for passionate, independent students but may be too much for struggling kids.
Skew the Script

Use real data from hot topics to make math relevant
Bottom Line: Easy-to-access curriculum supplement will spice up algebra and statistics courses.