Diagramming and Vector Drawing
Part of OpenOffice
http://www.lysator.liu.se/~alla/dia/ Recently integrated as a standard application in GNOME, dia is an excellent tool for drawing vector graphics. It can import and export all known file formats. Plugins are available to extend support for certain instances of diagrams (UML and SSADM are such examples). (ThomasAdam, January 2005)
http://www.xfig.org/ XFig has an extremely primitive user interface, and isn't the package to use if you want nice smooth, sweeping curves or fancy colour effects. However, if you can get used to the interface, it's a good package for doing outline "business" graphics. It can output (Encapsulated) PostScript, PDF, SVG, and assorted other formats. Its big bonus is that it can also output a split EPS/LaTeX format, with the lines in EPS, and the text (including !TeX formatting such as maths) in !TeX or !LaTeX format. This can be imported into !TeX, and the mathematical layout features of !TeX used in your diagrams. (HugoMills Mar 2004)
Not much in the way of text effects but an easy to use GUI
From the Inkscape web site:
Inkscape is an Open Source vector graphics editor, with capabilities similar to Illustrator, Freehand, CorelDraw, or Xara X using the W3C standard Scalable Vector Graphics (SVG) file format. Supported SVG features include shapes, paths, text, markers, clones, alpha blending, transforms, gradients, patterns, and grouping. Inkscape also supports Creative Commons meta-data, node editing, layers, complex path operations, bitmap tracing, text-on-path, flowed text, direct XML editing, and more. It imports formats such as Postscript, EPS, JPEG, PNG, and TIFF and exports PNG as well as multiple vector-based formats.
Written in python, sketch is probably the closest thing I've yet seen to ArtWorks/XaraStudio/CorelXara. It's still a long way away from being as feature-rich and as usable as ArtWorks, but it's a good start. (HugoMills Mar 2004)
I'm mentioning this as a bit of fun, but cadubi allows one to draw text-based images that can be displayed at the console.