A flash program created by Dr. Noob and Skipper Lee. Often abbreviated as "PMG". It basically has a ton of preset weapons and parts, and allows users to fully customize a weapon, create some sort of hybrid, or make a new gun entirely. It comes with many popular guns, such as the M4, AK47, M200, and many more, and has various weapon parts, ranging from simple barrels and flash hiders, to complex iron sights and fore grips.
There are two versions to the program, simply dubbed as the "Old" and "New" versions. The Old version has many more options, including shotguns, pistols, and various sub-machine guns, but the New version, while very complex and hard to master, offers more abilities to customize or create new weapons, especially the ability to change colors and modify the appearance of guns with shapes. An interesting note is that the user's imagination is the limit; some have created cars, tanks, ships, helicopters, and even people and characters, with only the program itself.
Because it's the same idea as playing with LEGOs, some look down on the program and it's users, for not having the skill to draw their own gun. This does not stop them, however, from faithfully creating new weapons, and recreating old ones, and showing them off in various internet groups, such as on Facebook or Deviant Art.
Look man, I made the Grav' Hammer from Halo 3 on Pimp My Gun!