GeoGebra
geogebra.org
Free maths software that combines geometry, algebra, graphing, spreadsheets and 3D in one tool, running in the browser or as an app.
Why it is useful
Lets you see how an equation and its graph or geometric construction are linked by dragging points and watching the algebra change. It covers school maths through to calculus, and teachers have shared a very large library of ready-made interactive worksheets.