I'm sure a lot of you have played this, but this thread is for those who don't know about it.
A deserted island... a lost man... memories of a fatal crash... a book written by a dying explorer. Dear Esther is a ghost story told using first-person gaming technologies. Rather than traditional gameplay, the focus here is on exploration, uncovering the mystery of the island, of who you are and why you are here. Fragments of story are randomly triggered by moving around the environments, making every telling unique. Features a stunning, specially commissioned soundtrack. Forget the normal rules of play; if nothing seems real here, it's because it may just be all a delusion. What is the significance of the aerial - What happened on the motorway - is the island real or imagined - who is Esther and why has she chosen to summon you here? The answers are out there, on the lost beach and the tunnels under the island. Or then again, they may just not be, after all...


This is my personal favorite Half Life 2 mod. It's really beautiful, would probably be described as art by most people. The writing is amazing, the music is incredible and the atmosphere is really immersive. It's been over a year since I've played this (came out 2008) and the experience has still stuck with me.
This has to be played, you will thank me.
http://www.moddb.com/mods/dear-esther/downloads