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:
Qalculate
gnuplot
GeoGebra
GeoGebra Portable
Advanced Trigonometry Calculator