Kivio is an easy to use diagramming and flowcharting application with tight integration to the other KOffice applications. It enables you to create network diagrams, organisation charts, flowcharts and more.
Dia is a gtk+ based diagram creation program released under the GPL license. It can be used to draw many different kinds of diagrams. It currently has special objects to help draw entity relationship diagrams, UML diagrams, flowcharts, network diagrams, and many other diagrams. It is also possible to add support for new shapes by writing simple XML files, using a subset of SVG to draw the shape.
It can load and save diagrams to a custom XML format (gzipped by default, to save space), can export diagrams to a number of formats, including EPS, SVG, XFIG, WMF and PNG, and can print diagrams (including ones that span multiple pages).
KFlowChart is a very young project intended to be a generic diagram maker. With it you can draw any kind of diagrams. This great flexibility is given by it's widget editor, with which you can design your own widgets.
Xfig combined with GNU/Linux and the GIMP were the only combination of tools I could get to do everything I needed for my fairly complex schematic.
Gnuplot. Famous scientific plotting package.
GtlGraph, a scientific graphing widget for GTK+V2. Capable of XY plots, polar plots and Smith Charts.
The GNU plotutils package contains software for both programmers and technical users. Its centerpiece is libplot, a powerful C/C++ function library for exporting 2-D vector graphics is many file formats, both vector and raster. It can also do vector graphics animations.
ploticus. A free, GPL, non-interactive software package for producing plots, charts, and graphics from data.