https://sciencevsmagic.net/

Nico Disseldorp fantastic website for generating beautiful and intriguing mathematical visualisations using fractals, tessellation and iteration.

https://www.mathler.com/

Mathler is Wordle for the numerically inclined. Suitable for children and adults, there is actually some quite interesting maths that goes into solving it in the optimum number of steps

https://mathigon.org/

The Mathematical Playground

Free tools, courses and manipulatives to make online learning more interactive and engaging than ever before.

https://brilliant.org/

For lifelong learners of any age, Brilliant replaces lecture videos with hands-on, interactive problem solving. It’s a better (and more fun) way to learn.

https://www.khanacademy.org/

Free to use, free to learn mathematics, science and many other subjects. Khan Academy set the benchmark and continues to expand into new subjects and languages.

https://www.desmos.com/

Intuitive graphing, available as an app. I use this tool all the time.

https://www.geogebra.org/

This is one serious maths teaching tool. With high-level functionality you can program a tremendous amount in.

http://www.3blue1brown.com/

Grant Sanderson’s famous channel where he calmly explains high-level topics with beautiful graphics

https://www.numberphile.com/

An incredible resource that not only teaches a tremendous amount of mathematics, but give a real insight into the people behind the maths