gPhoto is a program and library framework that lets users download pictures from their digital cameras. The libgphoto2 library gives you access to hundreds of models of digital cameras on several platforms. GIT repo and Issue tracker lives at https://github.com/gphoto/ these days.
Comix is a user-friendly, customizable image viewer. It is specifically designed to handle comic books, but also serves as a generic viewer. It reads images in ZIP, RAR or tar archives (also gzip or bzip2 compressed) as well as plain image files.
Sodipodi is general vector illustrating application for Linux/Unix and Windows. It uses W3C SVG as native file and in-memory image format and can do many neat things.
LinPHA is an easy to use, multilingual, flexible photo/image archive/album/gallery written in PHP. It uses a SQL database to store information about your pictures. It comes with a HTML based installer, so you don't need experience in setting up SQL
GImageView is a GTK+ based image viewer which supports tabbed browsing, thumbnail table views, directory tree views and drag and drop. It also support movies using the Xine library and MPlayer, and supports images in compressed archive formats.
Danpei is a Gtk+ based Image Viewer. You can look through your image files in Thumbnail form, and can rename,cut and paste them easily(like Windows Explore).
Java GUI that connects to content providers API such as Google, Bing, Wikipedia and implements a local search engine powered by Lucene, to search different contents: images, videos, articles, files and display them in an ergonomic OpenGL component.
DIOWave Visual Storage is a Web server which displays DICOM images. The look and feel of the Web pages are similar to imaging workstations.
A Final Fantasy XI model veiwer and exporter.
RIG is a web-based JPEG image album viewer, especially useful for digital camera albums; provides automatic image resizing, preview & thumbnail caching, user authentication; composed of a PHP web interface and a C++ thumbnail engine.