gldsrc 

An abbrieviation of GoldSource. It is a modified version of the Quake Engine and was used by ValvE, a seattle based games company, to create the award winning PC title Half-Life. It also served as the engine for Counter-strike and Team Fortress Classic.

There is also a large community that still works with the engine today (despite it being nearly 10 years old), creating maps and mods with custom made content.

NewMapper: "Can anyone tell me how to make a character speak to me in my map?"

VeterenMapper: "Source or Gldsrc?"

NewMapper: "Source"

VerterenMapper: "NO U"

gldsrc by Urban Nebula October 17, 2007