From the readme.txt:
Dear Esther is a Half Life 2 mod built as part of a research project by thechineseroom, a development team based at the University of Portsmouth, UK.
Rather than a traditional FPS game, Dear Esther is an interactive story, told using FPS technologies.
Using a stripped down version of the normal keys used for play, explore the island and unlock the story.
The story is randomised, so each visit will trigger a different selection of story fragments and reveal more details of the plot.
For more information about the project and other mods to be released, please visit: www.thechineseroom.co.uk
It was a genuinely beautiful experience.
There’s a professional version coming out that was previewed in the latest copy of PC Gamer. A must-buy for me.