Nightmare House is a single-player horror mod for HL2, made by Hen Mazolski, creator of Portal: The Flash Version Mappack, a very popular Portal add-on.
The first Nightmare House had you start in front of a creepy-looking house. You have to battle your way through zombies, puzzles, and other spine-tingling things to get out of the house. Two years later, Hen released Nightmare House: Remake. This time, you crashed your truck into a tree, and instead of going out into the road, you decide to check out the house nearby for help. However, you become trapped inside the new house, and you have to battle your way out again. NH:R also included a hint at a third Nightmare House, and Hen has been working furiously up to this point on it. Taken from the website: "The story resumes with the player lost and confused in a hospital known as "Never Lose Hope". With no understanding of how you came to be there, there's only a vague memory of the mysterious, haunting girl from Nightmare House. Soon it will be difficult to determine between a dream and reality. How long will it take you to realize that escape is not an option? "
NH2 requires EP2, so you need it to play. Also, Hen is including a remade NH:R in NH2 that doesn't require Half Life 2 (only requires EP2) or CS:S. If you missed out on NH:R because you didn't buy CS:S, now's your chance to play. And if you never played NH:R or NH1, there is no need to download it. There is a prologue in the game.
Some people claim this to be the "best mod ever" because of it's outstanding quality.
I however am too scared to play this after reading all the reactions.
Go check it out if you like horror games.