GeoGebra is dynamic geometry software. Constructions can be made with points, vectors, segments, lines, polygons, conic sections, inequalities, implicit polynomials and functions. All of them can be changed dynamically afterwards.
Elements can be entered and modified directly on screen, or through the Input Bar. GeoGebra has the ability to use variables for numbers, vectors and points, find derivatives and integrals of functions and has a full complement of commands like Root or Extremum. Teachers can use GeoGebra to make conjectures and prove geometric theorems.
(http://en.wikipedia.org/wiki/GeoGebra)
