Grapher!

Wolfram Alpha is a "Computational Knowledge Engine" - different from a search engine, it answers factual queries directly by computing the answer from structured data, rather than providing a list of documents or web pages that might contain the answer.

GeoGebra is free and multi-platform dynamic mathematics software for learning and teaching. It has received several educational software awards in Europe and the USA.

Gnuplot is a portable command-line driven interactive data and function plotting utility. It was originally intended as to allow scientists and students to visualize mathematical functions and data. It does this job pretty well, but has grown to support many non-interactive uses, including web scripting and integration as a plotting engine for third-party applications like Octave. Gnuplot supports many types of plots in either 2D and 3D. It can draw using lines, points, boxes, contours, vector fields, surfaces, and various associated text. It also supports various specialized plot types. »

matplotlib is a python 2D plotting library which produces publication quality figures in a variety of hardcopy formats and interactive environments across platforms. matplotlib can be used in python scripts, the python and ipython shell (ala MATLAB®* or Mathematica®†), web application servers, and six graphical user interface toolkits.

A beautiful, innovative, and modern online graphing calculator.

Online math solver, equation library, graphing calculator and science/engineering problem helper.Calculate multiple equations at once, Edit existing formulas and Create new ones, Compute large numbers of values, Plot graphs, Link your results, Solve full problems and Share worksheets with your friends.

Graph functions in two, three, and four dimensions, explicit, implicit, or parametric. Graph inequalities, contour plots, density plots and vector fields. Use rectangular, polar, cylindrical, or spherical coordinates. Solve equations numerically, graphically, or symbolically. Create mathematical movies and web pages.

Functy is a 3D graph drawing package. It allows Cartesian, spherical and parametric curve functions to be plotted and altered quickly and easily. This immediacy and the vivid results promote fun exploration of 3D function.

Core Plot is a 2D plotting framework for Mac OS X and iOS. It is highly customizable and capable of drawing many types of plots

GrafEq (pronounced “graphic”) is an intuitive, flexible, precise and robust program for producing 2D graphs and plots of explicit and implicit equations and inequalities.

Lybniz is a simple and very easy to use mathematical function graph plotter written in Python and GTK. Expressions are entered in Python syntax.