Data Visualization : Image Analysis

GIMP is a raster (as opposed to vector) image editor - it performs operations directly on the pixels in the image. It will support most basic image editing functions.
Platforms: Windows, Mac, UNIX

ImageJ is an open source Java-based image file converter and analyzer. There are many plug-ins available for scientific analysis.
Platform: Anything that runs Java

IrfanView is a free, simple, Windows-only image viewer and manipulator.

Seashore is a basic Mac OS X-based image viewer and manipulator. It is based on the GIMP technology and is likewise freely available.

GMT is an open source collection of ~60 tools for manipulating geographic and Cartesian data sets (including filtering, trend fitting, gridding, projecting, etc.) and producing Encapsulated PostScript File (EPS) illustrations ranging from simple x-y plots via contour maps to artificially illuminated surfaces and 3-D perspective views. GMT supports ~30 map projections and transformations and comes with support data such as GSHHS coastlines, rivers, and political boundaries.

Create maps interactively, draw locations, map boundaries. Output Postscript and Adobe Illustrator files.