The Zenautics Matrix Project is a matrix/vector class for C++. It supports real and complex data for nearly all matrix algebra calculations. It is optimized for the double type (std::complex). It works well with small and large matrices. It is easy to use and get started. It features plotting, FFT/IFFT, and more.
The TuxCap games framework is a GNU/Linux port of the PopCap games framework, which is used for professional 2D game development. It is written in C++ and supports both software and hardware accelerated drawing, comes with Python bindings, a very fast physics engine, a particle engine, a GUI widget system, and many documented examples to get you started.
libConfuse is a configuration file parser library written in C. It's goal is to be easy to use and quick to integrate into existing code. It supports sections and (lists of) values (strings, integers, floats, booleans, or other sections), as well as some other features (such as single/double-quoted strings, environment variable expansion, functions, and nested include statements). It makes it very easy to add configuration file capability to a program using a simple API.
Unified I/O is high performance Java I/O library. It features high performance (due to buffering), memory management, the ability to access streams, arrays, and files using the same interface, the ability to read or write directly from or into primitive arrays, and optimized reading over HTTP. Unified I/O also contains many utilities: Transformer, BitInputStream and BitOutputStream, PackBits, a RLE4 and RLE8 decoder, a base64 codec implementation, and many others.