Resources

Mod Kits & Game Engines

Unity3d 

Source SDK: Hammer 

Gears of War Editor

Unreal Development Kit

Radiant: Call of Duty 4 Mod Kit

CryEngine 3

Other Popular Game Engines

Level Design

Unreal Development Network

World of Level Design

Hourences

3dMotive

Chris Albeluhn

Wenda

UDK

The UDK Scriptures

Psychology & Ergonomics of Game Level Development & Asset Placement

Raven67854 

Unreal University

Phil Johnson

Micahpharoh

3D Buzz

3D Modeling & Texturing

Gnomon Workshop

Eat3D

Communities

Polycount

Gamasutra

ModDB

Gamebanana