Doom was the classic 3D shoot'em'up game from id Software. PrBoom is a game engine for playing Doom levels, which runs on both Windows and Linux/Unix systems. It uses the SDL graphics library. PrBoom a very faithful Doom port, with great effort taken to be compatible with as many original Doom levels and demos as possible.
PROJ.4 is a cartographic projections and datum shifting library written in C. It includes support for many (100+) projections, including Transverse Mercator and Lambert Conformal Conic. Included is a command line program for reprojecting points. It was originally written by Gerald Evenden of the USGS, and is in active use in various commercial and freeware software.
PVM (Parallel Virtual Machine) is a portable message-passing programming system, designed to link separate host machines to form a ``virtual machine'' which is a single, manageable computing resource. The virtual machine can be composed of hosts of varying types, in physically remote locations. PVM applications can be composed of any number of separate processes, or components, written in a mixture of C, C++ and Fortran. The system is portable to a wide variety of architectures, including workstations, multiprocessors, supercomputers and PCs.