GoldSrc

GoldSrc (also known as Goldsource) is a game engine created by Valve which debuted in 1998 with Half-Life. The engine is a heavily modified version of (and it also took a bit of code from other engines based on Quake).

GoldSrc added many new things from its predecessor.
 * NPC grouping capabilities.
 * Animating "bones" - Model vertices are grouped together into body parts which are then moved. This is as opposed to animating individual vertices.
 * Lightmaps with color.
 * NPCs capable of acting in scripted sequences.
 * Textures with alpha channels.