Graphics, Plotting and Data Visualization

gnuplot

gnuplot is a command line driven utility for creating 2-D and 3-D graphs. The official documentation is available here. The brief tutorial here may also be a good starting point.

The user mailing list is linked from the Help page.

XFig

Besides creating drawings in XFig, one way I find it really useful is in using it to process exported graphs from gnuplot. Plots created in gnuplot can be exported in a format called the fig format (among others). Once exported in this format, you can then open it in XFig, make any changes to it (including using LaTeX formulas) and export it to one of the supported formats.

Inkscape

Inkscape is a feature rich vector image creation tool. There are a number of tutorials for getting started and various mailing lists for seeking help and discussions are listed here.

dia

Dia can be used to create diagrams and supports exporting to a number of other formats. A mailing list is available here.

GGobi

GGobi is a visualization program for exploring high-dimensional data. Various resources for learning GGobi are listed here.

A mailing list is available for seeking help.

Mayavi

Mayavi is a software for 3-D data visualization. Documentation for getting started with Mayavi is available here.

Others

The GNU image manipulation program and g3data are other software available.