Open Tool Kit (Otk) is a portable widget library for making graphical user interfaces for C programs. It emphasizes simplicity for the application programmer without eliminating capability. Based on OpenGL, Otk supports Linux, Unix, and other OSs neutrally and efficiently. It is simple and compact, and it strives for easy compilation and linking to other applications. In seeking to address several issues associated with earlier graphics APIs, Otk explores some interesting methods, such as window-relative layout instead of pixel-based layout.
Mathomatic is a portable, general-purpose computer algebra system (CAS) that can solve, differentiate, simplify, combine, and compare algebraic equations, perform standard, complex number, modular, and polynomial arithmetic, etc. It does some calculus and is very easy to compile/install, learn, and use. The symbolic math application with a simple command-line interface is designed to be a colorful algebra calculator that is reliable, responsive, and convenient to use. The symbolic math library is lightweight and easy to include in other software, due to being written entirely in C with no additional dependencies.
Mapnik is a toolkit for developing GIS applications. At the core is a C++ shared library providing algorithms/patterns for spatial data access and visualization. Essentially a collection of geographic objects (map, layer, datasource, feature, and geometry), the library doesn't rely on "windowing systems" and can be deployed in any server environment. It is intended to play fair in a multi-threaded environment and is aimed primarily, but not exclusively, at Web-based development. High-level Python bindings (boost.python) facilitate rapid application development, targeting zope3, django, etc.
liboping is a C library for measuring network latency using ICMP echo requests. It can send to and receive packets from multiple hosts in parallel, which is nice for monitoring applications. Both IPv4 and IPv6 are supported transparently for the programmer and user. A program called oping is included to show the simplicity and potential of the library and provide the functionality at the command line.
InteLib is a library of C++ classes that lets you do Lisp/Scheme programming within your C++ programs, even without any additional preprocessing, without all those calling conventions. You can write code that is accepted by a C++ compiler while thinking in a "Lisp/Scheme mode", and the code you write will look much like Lisp/Scheme code, although it will be pure C++.