Gabes Love Tub	Home of the Missing Information mod and beginning of the Obsidian Mod.
Interlopers	Interlopers, the Number one resource for learning how to map.
Valve Developer Wiki	Valve Developer Wiki, Tutorials, etc..
Half-Life Improvement	Half-Life Improvement website. Specializes in making Half-Life and Half-Life 2 a better place.
Obsidian Conflict FileDB	Obsidian Conflict FileDB. Have a map and no hosting? Look no further. Sign up and download maps or upload your creations.