Dear Esther is a mod for Half Life 2 by thechineseroom. You can download it at http://www.fileplanet.com/196430/190000/fileinfo/Half-Life-2---Dear-Esther-Mod-v1.1, and if you have HL2, extract it into your SourceMods folder somewhere in SteamApps and run it.
If you can, do so now. It's short - maybe an hour to take it in slowly.
Note - There's a revised/updated version in the works, which might be worth waiting for. But speaking personally, I'd say try it now.
Dear Esther,
I sometimes feel as if I've given birth to this island-
Dear Esther isn't a game, really. There's pretty much no skill required to "beat" it, not even puzzle solving. Nor does it allow you much interaction with the world in the manner of a sandbox style of game. It's like if Mario had no enemies, no pits, no coins. Maybe one solitary block to jump between you are the end of the level. And it's ultimately entirely linear, though the level design is lacking and the path isn't always clear.
Dear Esther isn't a story, really. There's no coherent plot, no sense that you are seeing things in any particular order. What's on the screen depends on where you're looking. What narrative threads you're given are fragmented, in a random order, dispensed by an invisible narrator. The themes are mixed biblical, folklore, perhaps a little crazed science. It sets a mood, though. Even the flaws help this at times.
Though lacking suitable category, Dear Esther is art. That much is all but undeniable, though a certain degree of subjectivity is to be expected. Art is, I think, something that makes you feel - not neccessarily feel good. And Dear Esther is like having your psyche smashed out with a slice of heather wrapped around a chalk brick. And it wouldn't work - wouldn't even come close - if it were written or filmed instead, I think. Whether it has some meaning, some message - I don't know. Maybe.
I'd love to say more, but really, there's nothing else to say. There are no words, really. Even this much is too much. This thing, this game-that-is-not-a-game, is indescribably good. It defies description. And it is good.
-Come back!