BRL-CAD is a powerful constructive solid geometry solid modeling system that includes an interactive geometry editor, ray-tracing support for rendering and geometric analysis, path-tracing for realistic image synthesis, network distributed framebuffer support, and image and signal-processing tools.
ICI is a general purpose interpretive programming language that has dynamic typing and flexible data types with the flow control constructs and operators of C. It is designed for use in many environments, including embedded systems, as an adjunct to other programs, as a text-based interface to compiled libraries, and as a cross-platform scripting language with good string-handling capabilities.
Jhead is a command line driven utility for extracting digital camera settings from the Exif format files used by many digital cameras. It handles the various confusing ways these can be expressed, and displays them as F-stop, shutter speed, etc. It is also able to reduce the size of digital camera JPEGs without loss of information, by deleting integral thumbnails that digital cameras put into the Exif header. If you need to add Exif support to a program, this is a simple program to cut and paste from. Many projects, including PHP, have reused code from this utility.
lcra renames filenames like COMMAND.COM and MAIN.C to more tasteful ones like command.com and main.c. It tries to guess which filenames are DOS-ish ones needing changing, and which are really meant to be uppercase. It will fix the contents of a directory and all subdirectories. The companion program nsra changes spaces in filenames to underscores.
Pip is a wrapper for programs that won't use stdin and stdout, but require filenames to be given on the command line; it lets you use '-' as a special filename. Now you can build pipelines to your heart's content. Pip takes care of creating and removing the temporary files needed. Also included is pip_latex, which handles the peculiarities of TeX and LaTeX.
SQLite is a small, fast, embeddable SQL database engine that supports most of SQL92, including transactions with atomic commit and rollback, subqueries, compound queries, triggers, and views. A complete database is stored in a single cross-platform disk file. The native C/C++ API is simple and easy to use. Bindings for other languages are also available.
TEA Total is a collection of extremely small encryption tools. At the heart of TEA Total is the TEA (Tiny Encryption Algorithm): a fast and secure 128-bit private key algorithm which was developed and placed in the public domain by David Wheeler and Roger Needham of the Cambridge Computer Laboratory.
mtail is a small tail workalike that performs output coloring using ANSI escape sequences (although the sequences are overridable, so you could cause it to output something else, e.g., HTML font tags). It is written in python, is fairly small, and should be relatively platform-independent.