Video Game Mods

A video game mod is an addition made to an already released game usually made by fans that add new worlds with their own levels and mechanics. In almost all cases, the original game is required to play mods, though some become popular enough to become their own things.

One of the most well known examples is Garry's Mod, which itself is designed around creating mods, and is also a mod itself made from Half-Life 2.