Flexible and free mathematical software
GeoGebra is a complex mathematical application for those that study or work with arithmetic, geometry, algebra and calculus.
Its a rather complex application that's aimed strictly at those comfortable with complex maths but the advantage GeoGebra offers over similar apps is that it provides multiple representations of objects that are all dynamically linked. The idea behind GeoGebra is to connect geometric, algebraic, and numeric representations in an interactive way.
This can be done with points, vectors, lines, conic sections. GeoGebra allows you to directly enter and manipulate equations and coordinates enabling you to plot functions, work with sliders to investigate parameters, find symbolic derivatives, and use powerful commands like Root or Sequence.
However, the complexity of the program is mind boggling for those new to such mathematical applications. It's very difficult to work out how to use from scratch although there are several very detailed tutorials to help you on your way.
GeoGebra isn't a mathematical program for the faint hearted but if you have to deal with arithmetic, geometry, algebra and calculus on a regular basis, its very flexible.