CAILogin (Counter AI Login) is an image-based user verification program for web-page logins. It's written in plain C with no special dependencies, and hence it's very suitable for embedded systems which lack "big" interpreted languages, such as PHP.
img2xhtml is a little and fast ansi C program that converts images (jpeg for example) into plain xhtml files, that draws the image using html elements to draw the divs, there is no difference between the original image and his xhtml version.
Command-line tool for manipulation with PPM images. It can create a single image from multiple images by mixing them on top of each other. It can also zoom and crop image, or perform all of these operations simultaineously.
tinydot is a small programm for translating an abbreviated into the original dot language. Using plugins for wiki systems it is possible to create directed graphs in a wiki manner.
Ilib is a library (with tools and examples) written in C that can read, create, manipulate and save images. It can read and write PPM, BMP, XPM, GIF, PNG and JPG image format.
An Open Source generator of "Turing images" (also known as "CAPTCHA") which explores the use of different mathematical models like fractals, noise generation and the Fibonnaci sequence, rarely used in this field.