Links

LEVEL DESIGN:
Q3radiant Help
Bubba's Arena Mapping
Mapping 4 Newbies
General Mapping Tutorials
RTCW/ET Mapping Tutorials - all apply to Quake3 mapping

GUIDES:
Scripting Guide

EASYGEN:
Easygen Website
TramDesign Easygen Tutorial
Modsonline Easygen Tutorial

OPTIMIZATION:
Advanced Portal and Hinting Optimization Tutorial
Brainerd Hint brushes Tutorial
Understanding Vis and Hint Brushes

EN FRANCAIS:
Débuter avec GtkRadiant - A tutorial in French for getting started with GTK Radiant. Covers the basics: Rooms, corridors and doors. (Thanks Dexx)
Optimisation : Les Leafs et les Hint brushs