For the people who are wanting to create their own maps, but don't yet know how to make them; here is a list of tutorials.
Green = Beginner
Orange = Intermidiate
Red = Advanced
(Optional) setting up the editor for first time use
Basic mapping tutorial (Creating rooms, lights, enemies...)
Basic door creation
Creating triggers that will activate certain objects
Creating realistic sunlight
Creating a good looking terrain for your map
Gravity around certain objects
Creating rain particles for rainy effects
A way of teleporting moving brushes
Creating cut scenes for your single player maps
Some nice video tutorials, they helped me A LOT
Plus, something I found just now:
AngryPenis.org SEd tutorials
For the official SED wiki tutorials, click here. Edit: These tutorials are no longer available.
Ignore the Warning: include(); things, they will not disturb your reading on the tutorials, does it?