L4D Level Design/Outdoor Levels - Valve Developer Community
Canyon Terrain Editor | Valve Developer Union
Is Nem's Tools Terrain Generator any good? : r/hammer
CS:GO SDK Hammer Source Tutorial - Abandoned House Workflow (Part 4/5) - Displacement/Terrain and Foliage
Tile Editor Basics - Valve Developer Community
I've been working on making large-scale custom terrain with heightmaps and megatextures using worldmachine. This is my latest one, I think it turned out pretty good! : r/hammer
Wiki: Tutorial: Advanced Terrain Creation - TWHL: Half-Life and Source Mapping Tutorials and Resources