This is an area designated for galleries of Custom .MDL Files. In Quake, a .MDL file is the file containing the models and animations of the various entities in the game, plus the textures utilized. Essentially, this is where the cosmetic changes to models occur.
.MDL files control the appearance of Monsters and Ranger. All 3d (2d are sprite files, or .SPR) effects and decorations can be found here, including things as simple as a torch. It is also responsible for the appearance of the various Powerups and Weapons, but not the Ammo which is treated as a .BSP file. This is because Ammo and the 15 Health, 25 Health, and 100 Health are simple boxes, which could lead to distortion effects if attempted to be made into a .MDL, and thus were treated as .BSP as those models can easily handle being textured while being boxes.
This page is intended to catalog the various .MDL changes or additions out there. This is not for new entities, all files here feature obituary/pickup message that remain unmodified and thus the illusion of it being a new thing is broken by Quake stating it is still the original entity.
Note that these .MDL files may be compatible with only one port. For example, many of the 1996 .MDL files were designed to run in DOS and thus might not work properly in a modern Source port. Usually, the files will work if it is a reskin, but changes to the monster shape will result in the game crashing. Your mileage may vary; these are specifically designed for the original DOS Quake executable.
Also note that any files made prior to June of 1996 most likely will not work with Quake. Qtest files are entirely incompatible with the final product and are thus not documented here. Instead, they can be found on the Qtest Custom Files page.