A Half-Life modification: Afraid of Monsters DC
Probably one of the best mods for this game, and in my opinion the best one. The game is about a guy addicted to painkillers, who lately experiencies horrendous allucinations. seeing as he cant stop taking them, he seeks help at a hospital, making a stop at the bathroom, were a bottle of pills awaits him and marking the beggining of the nightmare. The game is pretty scary, full of small details to scare you. Also the game like you to travel into pitch black areas, relying on your flashlight (with has limited battery, but you can get more). In this game, any light source and are your only chance of survival. I hope that covers the basic idea of the game. I should point that this games is really hard, and having no flashlight battery in the middle of darkness is really frustating (your flashlight recovers a little battery if you wait long enough), however there are very small sources of light here and there to guide yourself (like looking at the treetops in a forest) Also, this mod has 4 different endings. EDIT: here is a small trailer http://www.youtube.com/watch?v=xfpIo63Ijj4

Agreed.
I can imagine anyone into K7 would also potentially dig this Mod. It's very well thought out, has a classic K7 'wtf??' vibe to it, and is highly pant-shredding in several places. ("Hmm I wonder what happens when I flick this switch off in the basement..") Apparently the same guy's now working on a new mod, which also looks great: http://www.moddb.com/mods/cry-of-fear *sudden weird head wobbling* Arrrgg!! *ahem* sry. It's not just the dark, it's what lurks in there.. W

Yeah its fun since all those scary things find keep you on your toes, specially since monster can kick your ass so easily, leading you to get really paranoid about everything you see. It really makes you plan ahead and memorize stuff. Different ending really added replay value (first HL mod with that) and different paths, not to mention the "real" ending leads to an awesome room.