Blast's 3D gaming and editing page
Information and tutorials about how to create 3d game levels and mods specific for this game
Half-Life Editing Resource Center
Resources for level designers including tutorials, tools, and forums.
Half-Life SDK2
Resources on how to program add-ons using the SDK2 with C++.
Half-Life Workshop
Tutorials and resources for level designers.
Wavelength
Tutorials and information covering level design, coding, modeling, and sound creation.