In an effort to prevent further dog shit Half-Life 2 maps, Leak Free has come up with a guide to good level design. The online tutorial includes to basic introductions to level design, a walk-through for making a two room, one hall map (that s about my level of ability) and how to create a sunset. You could always run out and buy the Official Mod Maker s Guide by Prima, but this is free and better than any FAQ on the subject.

Tutorials [Leak Free, via Joystiq]