Mathigon
Interactive textbook and tutor combined, making learning math more personal, fun, and interactive.
Steve Wyborney
Steve Wyborney is an award winning American educator, well known for his work with mathematics. He is the author of 25 Common Core Math Lessons for the Interactive Whiteboard, Week by Week Math Review for the Digital Classroom, and Long Division Made Easy for elementary grades.
Estimation
https://stevewyborney.com/2018/04/the-estimation-clipboard/
https://stevewyborney.com/2019/09/51-esti-mysteries/
Nrich
Nrich is a collaborative project undertaken by the Faculties of Math and Education at the University of Cambridge, providing thousands of free online resources for students ages 3 to 18.
Would You Rather Math
Engaging students in conversations about math and having them justify their ideas or reasoning using math.