Ghostscript is a processor for PostScript and PDF files. It can rasterize these files to a wide variety of printers, devices for screen preview, and image file formats. Since applications tend to prepare pages for printing in a high-level format such as PostScript, most Unix users with low-level bitmap printers, such as inkjets, use GhostScript as part of the printing process. In addition, Ghostscript is capable of converting PostScript files, functionality comparable to Adobe Acrobat Distiller, but on the command line. In addition, Ghostscript is used for file import and viewing by a great many other applications, including xv, ImageMagick, gimp, and xdvi. Several GUI wrappers for viewing PostScript and PDF files exist, including GSview, ghostview, gv, ggv, and kghostview. This is far from a comprehensive list.
Allegro is a multi-platform game library for C/C++ developers that provides many functions for graphics, sounds, player input (keyboard, mouse, and joystick), and timers. It also provides fixed and floating point mathematical functions, 3D functions, file management functions, compressed datafile, and a GUI.
Tux Paint is a simple and entertaining drawing program geared towards young children. It has a simple interface, sound effects, and a cartoon character (Tux, the Linux penguin). Along with drawing brush strokes, lines and shapes, you can also enter text and place "rubber stamp" (or "sticker") images on the picture. Tux Paint is extensible, and could be useful in an educational environment (such as a grammar, elementary, or grade school). It's portable across numerous platforms, and runs well even on slower systems like the Pentium 133MHz.
PECoMaTo stands for Picture-Embedded COntents MAnipulation TOol. It is basically designed to display any kind of information embedded in picture files, as well as checking, filtering, extracting, removing, adding, and fixing such information. In other words, it's a metadata processor. It supports the following file formats: JPEG/JFIF, Adobe PSD and FFO, and raw IPTC. It knows about the following metadata formats: JFIF, IPTC, Exif, Adobe, and Fotostation. More file and metadata formats might be supported later (TIFF, etc.).
Grafx2 is a bitmap paint program that allows you to draw in more than 60 video resolutions (from 320x200 to 1024x768, including most of the standard Amiga resolutions: 320x256, 320x512, 640x256, 640x512, and more, provided your video card knows how to handle them). Its layout is not very different from the famous Deluxe Paint or Brilliance, so it will be quite easy to handle it if you know at least one of these programs.
Pixel is an application for editing, retouching, manipulating, and animating RGB, CMYK, and HDR images. It was formerly known as Pixel32. It is available for Windows, Linux, Linspire, Mac OS X, BeOS, Zeta, QNX, MorphOS, FreeBSD, eComStation, OS/2, SkyOS and even old plain DOS, for both x86 and PowerPC architectures.