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++.
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.
libjson is a JSON parser and printer that is fully compliant with the JSON spec. The parser is efficient and interruptible with a simple and flexible API. It also optionally supports extensions like C-style comments or Python/YAML style comments, and for security allows the user to set a limit on the number of nested structures and the parse buffer. The library plugs easily into any model since it doesn't include any object model, and plugs easily in any project's memory management since the memory functions can be redefined.