GeoGebra

From the website:

GeoGebra is a dynamic mathematics software for education in secondary schools that joins geometry, algebra and calculus.

On the one hand, GeoGebra is a dynamic geometry system. You can do constructions with points, vectors, segments, lines, conic sections as well as functions and change them dynamically afterwards.

On the other hand, equations and coordinates can be entered directly. Thus, GeoGebra has the ability to deal with variables for numbers, vectors and points, finds derivatives and integrals of functions and offers commands like Root or Extremum.

These two views are characteristic of GeoGebra: an expression in the algebra window corresponds to an object in the geometry window and vice versa.

I would characterise the website as a tool for creating visualisation of mathematical equations. :-)

See examples and an article Creating Mathlets with Open Source Tools