This is an amazing collection of hundreds (thousands?) of well-designed Geogebra worksheets:
The author, Daniel Mentrard, has put together a huge library of mathematics and physics demonstrations/explorations that are all available for free.
Although the site is in French, it’s not too hard to browse the many Geogebra resources covering Arithmetique, Art et Maths, Algebre, and much more.
And if you like, you can always run the website through Google Translate.