I've been watching Helloween LP this, and it's surprisingly well made for a mod, Dodgy A.I, and inventory problems aside.Sarah Kerrigan said:Cry of Fear for the original Half Life.
My god. That Mod. No words could tell you how terrifying it is. Just..play it.
Also, it's scary, very scary. That chainsaw guy in the woods absolutely terrified me, as well as a number of the enemy designs.
I would definitely play an updated version using the Half Life 2 source engine, if they ever make one.