This was Diane's compilation of WET and Quake3 mapping links.
Now (speaking 27.03.2010) it is an updated list with hopefully links to everything that is still out there.
General Information:
Basic information about the engine / mapfile concept
Basics of the editor
General tutorial
Beginner to pro tutioal (et)
A complete set of tutorials covering nearly everything (german)
Et specific information from the editor
Specific information:
Compiling process / fps optimization / lights
Creating Rubble and Ruined Walls
Terrain techniques / clipping
q3map2 (compiler) switches
Shadermanual (the mappers bible
)
Basic mapscripting tutorial (pdf)
Mapscripting tutorial (pdf download)
TC:E specific tutorials:
Setting up the mapping environment
Creating custom skins
Creating custom weapons
Tools / files:
"nightly" radiant builds
md3 viewer
TC:E specific files:
unofficial mdk (made by liquid)
TC:E entity definition file
Further information
Bsp file structure
Again bsp structure
Other links:
Splashdamage et editing forum
Gtk radiant official page