Here are some links to other programs useful, in one way or another, for desktop publishing sorts of tasks. The Inkscape and Scribus sites in particular both have many useful links to such things, and lively and informative discussion lists and wikis.

Indispensible Artsy Software

The Gimp
Image editor, since 1996! Huge user base, and tons of plugins available.
A rapidly evolving image editor, roughly in the same category as the Gimp, but with more of an emphasis on painting. Features many exciting tools and abilities such as wraparound editing, symmetry editing, color mesh, mesh transform, and robust brush configurability.
Although Blender is primarily for 3D modeling, there are a number of features that can be used for 2D design, such as the grease pencil, and the Freestyle rendering type. Also extremely useful is Blender's modifier stack, which makes it very easy to make complex shapes out of a few operations. Laidout's node system is heavily inspired by Blender's nodes, as well as the Animation Nodes addon for Blender.
Somwhat new 2D animation software, with some very neat and easy to use features, such as morphing paths by marking vertices to "dissolve", other fancy path distortion effects, as well as ability to directly use fragment shaders.
The go to open source vector graphics editor based in svg. Very active development, tons of filters and extensions. Some day, even animation.
Amazing software for drawing. Minimal interface, but powerful brush engine.
Imagemagick Utilities
Command line tools to do a plethora of image tinkering.
Graphicsmagick Utilities
Command line tools Very similar to Imagemagick from which it forked, but implemented differently in places.
Capable DTP, with image caching for image heavy documents. Extensive PDF abilities. In terms of functionality, what's missing is impositioning.
A powerful font creation program. If you want to make fonts with open source software, for a very long time, this was your only option. These days, there are a variety of emerging contenders like Metapolator and Trufont. Be sure to set aside a fair amount of time to learn the interface.

Other Artsy Software

In addition to the Indispensible stuff above:
The quintessential, very powerful layout engine, though admiration for it's flexibility and sophistication might be tempered by the fact that it works from text configuration files, so good luck with the learning curve!  
An image effect processor. Mass edit photos from your camera to get color settings more to your liking.  
Raw Therapee
Another image effect processor, similar in use to Darktable.  
A set of tools to help process PDF files. Podofoimpose, for instance, imposes PDF documents using a plan file based on a minimal lua based syntax. There is also Podofobrowser, which is a great low level browser of the contents of PDF files.  
Kind of a graphical front end for TeX/LaTeX  
Forked from Openoffice in 2010, when Oracle bought Sun. Most of the OpenOffice devs switched over to LibreOffice. Oracle later sold off OpenOffice to Apache.
Calligra Words
Formerly Kword, an easy word processor. MUCH smaller than Libreoffice. Its file format these days is odf, like LibreOffice.
(pronounced pus-par-too)
Exemplary straightforward gui for simple wrapped text and images. Very simple and easy file format. Has phenomenal, unmatched "Inspiration" button that lets you play a mini-game of worms. It is very important for software to inspire the proper work ethic, and the Inspiration option does that admirably by allowing you to play a mini-game of worms! No longer in development since 2007.
Command line utilities to perform various things like booklet impositioning on postscript files
Relatively simple java based programming environment that greatly streamlines creation of strange tools as well as animation.
Another programming environment for visual elements. This one for python lets you write code to generate lines, shapes, pull in images, and flow text in areas with pleasant line breaking, across multiple pages! Flat is the python side, and Even is the real time viewer.
Kind of a last step before the printer actually prints, this is a virtual printer that takes a print job and can rearrange into booklets.

The Big Bucks

Adobe InDesign, (wikipedia)
Adobe Pagemaker
(formerly Aldus)

Development abandonded in 2004
Adobe Freehand
(formerly Macromedia Freehand, formerly Aldus)

Development abandonded in 2007
So-Far-Not-Adobe CorelDRAW
Has imposition features!, (wikipedia)
So-Far-Not-Adobe QuarkXPress, (wikipedia)

The Small Bucks

Has existed since 1985, runs on even Amigas and Atari!!
Xara Xtreme
Vector graphics. It appears they gave up on the free Linux port!

      Other Links

You might check out Libre Arts, which keeps tabs on the latest in open source graphics software. Also watch out for the annual Libre Graphics Meeting, which brings together users and developers from all over the world to hash out what's happening and what's going to happen in the field.

Here are lots of links from the Laxkit for various specifications and programming libraries: