Trolltech Qt Platform Independent GUI - http://qt.nokia.com/
MATLAB (commercial) - http://www.mathworks.com/
Visualization Toolkit (VTK) and Insight Toolkit (ITK) http://www.vtk.org/ http://www.itk.org/
ImageJ (opensource) a Java version of NIH image - http://rsb.info.nih.gov/ij/
VisiQuest/Khoros (commercial+academic) - http://www.accusoft.com/
Interactive Image SpreadSheet (IISS) - http://meru.cecs.missouri.edu/mvl/iiss/users_manual/
OpenDX Data Visualization Explorer (formerly IBM DX): http://www.opendx.org
IDL - Interactive Data Language (commercial): http://www.ittvis.com/
SGI ImageVision library http://www.sgi.com/software/imagevision/
Image Processing tutorials using VisiQuest http://www.accusoft.com/support/evalcenter/tutorials/dip/VQ/index.htm
VisiQuest Presentations (in Macromedia) http://www.accusoft.com/imaging/visiquest/vq_presentations.asp
Java-based image processing tutorials at Univ of Edinburgh (HIPR2) http://homepages.inf.ed.ac.uk/rbf/HIPR2/hipr_top.htm
Nice pictures at Kodak Digital Learning Center http://www.kodak.com/US/en/digital/dlc/book3/chapter1/ Introduction to digital imaging from the Eastman Kodak Company (the original web pages seem to have moved). There are five lessons with review questions and competency exams, covering fundamentals, image capture, and processing. Good overview of color theory.
Digital Photography Imaging glossary http://www.dpreview.com/learn/Glossary/Digital_Imaging/
Multimedia signal processing links http://www-ee.stanford.edu/~gray/iii.html
Introduction to Digital Signal Processing http://www.bores.com/courses/intro/index.htm Includes overview of DSP, converting analog signals to digital signals, aliasing, frequency resolution, and quantization error.