GoldSrc
Goldsource (also known as GoldSrc), is the heavily modified version of the Quake engine that Half-Life runs on. For information about converting GoldSrc content to the Source engine, see Porting Goldsource content to Source.
Features
AI flocking - NPCs can group together, seen with Houndeyes
Skeletal animation - allowing for easier animating of models
Colored lighting - Quake simply had monotone lighting
Scripted sequences - Used extensively to tell Half-Life's story
Transparent textures - Glass can now be used in maps
Higher poly counts - GoldSrc allows for far more detailed models than Quake
SDK
The GoldSrc SDK is known as the Half-Life SDK. As of August 2013, it is available on Steam under the "Tools" section. The code can be found on Valve's Github Page