The Source engine is a 3D game engine developed by Valve Corporation. Its unique features include a large degree of modularity and flexibility, an artist-driven, shader-based renderer, accurate lip sync and facial expression technology, and a powerful, efficient and completely network-enabled physics system.
It's difficult to phrase it without sounding like I'm fanboying, but Source is the best game engine out there. The most important reason for this is organization.
The engine, engine branch, game, mod, and SDK file directories are very well organized.
Engine Branches: Half-Life 2 and the episodes all use the same engine branch, so they all share a directory.
In this case, the engine branch directory is
And the game directory for Half-Life 2 would be
The basic structure of a game's directory in one engine branch is virtually the same with all engine branches
If you find the file "gameinfo.txt" in:
Then you will most likely find a similar file in:
steam/steamapps/common/left 4 dead/l4d/
If you learn to mod one source game, then you also learn how to mod most of Valve's games so far, and probably most of Valve's games in the future.