Elements of War: Source SDK

Software Development Kit

Gold Source ENGINE and Source ENGINE

Half-Life / Half-Life 2 / All Valve Titles

Gold Source SDK / Half-Life SDK

http://www.fileplanet.com/81535/80000/fileinfo/Full-Half-Life-SDK-2.3

Source SDK / Half-Life 2 SDK

SteamPowered.com

Start Steam, "Library", then click, "Tools", find Source SDK.