GeoGebra facilitates the creation of mathematical constructions and models by students, for learning and teaching at all levels of education. It allows interactive explorations by dragging objects and changing parameters, joining  interactive 2D and 3D geometry, algebra, tables, graphing, calculus, and statistics.  GeoGebra is also an authoring tool for teachers to create interactive worksheets, and it is Open Source software, freely available from Download the offline version of the book here