Written and maintained by the open source community. A portable and highly efficient TrueType rendering engine.
http://freetype.sourceforge.net/index2.html
Written by Jean-loup Gailly (compression) and Mark Adler (decompression). As quoted from the website, ZLib is "a massively spiffy yet delicately unobtrusive compression library."
IJG is an informal group that writes and distributes a widely used free library for JPEG image compression.
The official PNG reference library
http://www.libpng.org/pub/png/libpng.html
A set of C/C++ and Java libraries providing Unicode and Globalization support for software applications.
GLEW was developed by Milan Ikits and Marcelo Magallon.
The OpenGL Extension Wrangler Library (GLEW) is a cross-platform open-source C/C++ extension loading library. GLEW provides efficient run-time mechanisms for determining which OpenGL extensions are supported on the target platform. GLEW has been tested on a variety of operating systems, including Windows, Linux, Mac OS X, FreeBSD, Irix, and Solaris.
A C++ library that takes GLSL shaders, does GPU-independent optimizations on them and outputs GLSL back.