DESMOS - Graph functions, plot data, evaluate equations, explore transformations, and much more
https://www.desmos.com/

History of Mathematics (includes chronology and biographies)
https://mathshistory.st-andrews.ac.uk/HistTopics/

Biographies of Women Mathematicians
http://www.agnesscott.edu/lriddle/women/women.htm

Introduction to Fractal Geometry
https://pardesco.com/blogs/news/fractal-geometry?srsltid=AfmBOorRWvEZkLuSLFg4UZ0BGiDvPimFXwEZ4oIbg19yxKEy6ZCwkr82

What is the Mandelbrot set?
https://plus.maths.org/content/what-mandelbrot-set

Khan Academy
https://www.khanacademy.org/math

Purple Math
http://www.purplemath.com/modules/index.htm