Source SDK

Source SDK is a way for mod makers to easily create mods, animations and maps on the Source engine through this tool.

Faceposer

Faceposer is a tool which allows the user to create custom facial animations and body movements of characters that are used in games that are powered by the Source Engine. It's useful when making Machinimas or Mods because it lets the user Lip Sync the characters so that their mouth movements match the recorded audio dialouge they are saying in the Machinima or Mod.

Hammer

Hammer is the level design tool used by Valve to create the various levels in their games, it is just like Hammer for GoldSrc used for the original Half-Life however it is update with more tools and entities so that it is compatible for the Source Engine. Anyone who owns a game powered by the Source Engine has access to Hammer and can easily learn to create maps therefore there are some huge Source level design communities.

Model Viewer

Model Viewer allows developers view their finished animations made in Faceposer or any other animation program. It also lets them view props, buildings and weapons however it cannot view anything created in Hammer or textures. 
 

Trivia

• The Half-Life 2 engine and the Orange Box engine are categorized seprately due to some differences, so maps created for Half-Life 2: Episode 2 will not run in Half-Life 2 or Half-Life 2: Episode 1.