PLIB is a suite of free and open-source libraries to ease the development of computer games. It was originally written by Steve Baker in 1997 and licensed under the LGPL.
PLIB includes sound effects, music, a complete 3D engine, font rendering, a simple windowing library, a game scripting language, a GUI, networking, 3D math library and a collection of utility functions. All are 100% portable across nearly all modern computing platforms. Each library component is fairly independent of the others to encourage replacement with other libraries like SDL or FLTK.
PLIB has been used by many projects (not all games, and not all open-source), but has not kept pace with technical developments that happened after its original conception (e.g., shader technology), so it is currently outdated and rarely used for new projects. While nowadays there are better portable and open-source replacements for all of its components are available, a similar comprehensive suite of libraries is still missing.