PhysicsFS
PhysicsFS (PhysFS for short) is a zLib-Licensed programming library written in C for virtual file access. In the 1.x series, it maintained a global search path that allowed reading from normal directories, ZIP files, Quake PAK files, Doom WAD, which would all be represented in a single virtual file tree with one directory designated as where output files are to be written. In the newer 2.x series, a more UNIX-like mount-based system is used, in which directories and archives can be mounted as specific directories as supposed to part of the root tree. For example, one could mount graphics data files from Graphics.WAD and script data files from Scripts.PAK as root directories, the user directory for the game as "/saves".
Usage
PhysicsFS is mainly used in the video game industry, but can be used by other sorts of programs, too. The Allegro and LÖVE game engines support it natively. Video games that use PhysicsFS include Dead Meat, netPanzer, and MindBender.