If you are an advanced level editor, you'll probably find what you need though. These guys used to have a lot of tutorials, but now they've moved toward frequent news updates on the world of HL editing. They also have some resources worth checking out, such as information on game's dimension units and some others. A large Half-Life editing site whose strong point is that it has several departments. They include level editing, sound, 2D design, coding, and more.
Updates seem to have stopped, but the archived content can still be quite useful.
The Handy Vandal's Almanac A quite amazing resource center for Half-Life designers. The site collects links to tutorials on different topics from different Half- Life sites, and by selecting a certain topic, you get a whole listing of tutorials on it. The guy who runs the site is incredibly dedicated, and the site is always neat and up to date. Every aspiring mapper should have this page bookmarked. This site has a lot of tutorials on basic mapping, which can be useful to beginners.