Allegro is a multi-platform game library for C/C++ developers that provides many functions for graphics, sounds, player input (keyboard, mouse, and joystick), and timers. It also provides fixed and floating point mathematical functions, 3D functions, file management functions, compressed datafile, and a GUI.
DeuTex is a wad composer for Doom, Heretic, Hexen and Strife. It can be used to extract the lumps of a wad and save them as individual files. Conversely, it can also build a wad from separate files. When extracting a lump to a file, it does not just copy the raw data, it converts it to an appropriate format (such as PPM for graphics, .au for samples, etc.). When it reads files for inclusion in pwads, it does the necessary reciprocal conversions. In addition, DeuTex has functions such as merging wads, etc.
Slash'EM (Super Lotsa Added Stuff Hack -- Extended Magic) is a variant of the hugely popular roguelike game NetHack. It adds more monsters, more objects, more dungeon levels, more roles, and more races, shopkeeper services, and techniques for you to use, and invisible objects and gypsies for those who feel lucky. It also comes with a GTK windowing interface and higher resolution tile sets as options.