Unfortunately with the age of Half-Life/GldSrc engine. ALOT of old links containing many great tutorials and files are obsolete. Thanks to old hosts like Gamespy, Gamedesign, Planetside, Telefragged, Geocities, etc for being utter shit. If you come across old links while scouring the web, just put them in the Wayback Machine : https://archive.org/web/. It’s the only advice I have for you.
Help & Tutorials:
Sourcemodding: https://www.sourcemodding.com/tutorials/goldsrc/
The 303 Tutorials: http://www.the303.org/tutorials/
Mod DB Forums: https://www.moddb.com/forum
The Wavelength: http://articles.thewavelength.net/
Tommy’s Hammer Tutorials: http://www.slackiller.com/tommy14/tutorials.htm
EyeRoniK Hammer Tutorials: http://www.slackiller.com/tutorials/tutorials.htm
HL Improvement Forums: http://hl-improvement.com/
Prefabs, etc:
Zoner’s Half-Life Tools: http://zhlt.info/
Snarkpit (Currently Down): http://www.snarkpit.net/
Gamebanana: https://gamebanana.com/prefabs
The Whole Half-Life: https://twhl.info/
Slackiller: http://www.slackiller.com/index.htm
Goldsrc Warehouse (Gamer Lab): http://gamer-lab.com/eng/goldsrc
GVME.org: https://www.gvme.org/cat/15