Maxima is a system for the manipulation of symbolic and numerical expressions, including differentiation, integration, Taylor series, Laplace transforms, ordinary differential equations, systems of linear equations, polynomials, sets, lists, vectors, matrices and tensors. It is a free and open source function plotting application that's straightforward, light on system resources and advanced.
Here are some screenshots to illustrate the features and user interface of this application:
GeoGebra
GeoGebra Portable
Qalculate
gnuplot
Wabbitemu