Collect them all here and then distribute to the appropriate sections, once they exist.
Huge resource for Source information, and home of the official SDK documentation.
Basic user-written tutorials and developer news.
Tutorials for mapping, modelling and texturing.
Articles and guides about several modern game engines, including HL2.
Basic mapping tutorials.
Zombie's little corner of rarely visited webspace
Some slightly... ghetto tutorials and a bunch of tools.