GeoGebra is a free award-winning downloadable software program and apps that combine geometry, algebra, spreadsheets, statistics and calculus. For example, you can use it for geometry to construct objects from points, vectors, segments, lines, and conic sections, and then use functions to change the objects dynamically. Use it to find derivatives and integrals of functions and it offers commands like Root or Vertex.
There is extensive documentation on the software, a large community of users, online forum, and many online videos to assist you. Click the Team link on the site to see examples of Geogebra in action. And there’s a growing collection of app versions of many of its components, including a graphing calculator, for iPad/iPhone and Android devices.