Dear Esther is part of a research project, creating experimental game mods, based at the University of Portsmouth, UK. The aim was to use Source to create something radically different from normal: an interactive story that dispensed with traditional gameplay and focused instead on an open-ended, semi-random narrative.
I can't recommend this enough.
In this mod you wander at a walking pace around a gloomy Hebridean island, finding clues which reveal the history of the island and yourself. The voice acting is superb and atmosphere is perfect.
It is most amusing to see the reactions to this on Moddb - most are complementary, but naturally there are many who say "no guns? lolwhut?"
quite odd, slightly unsettling (especially the noises). certainly the most poetic thing I've ever seen done in HL2. The caves especially were special.
(trying really hard not to give anything away, this is a mod to play when you're feeling like a bit of alone time, oh, and for those still on the fence I'd say it'll take you less than 45 mins, but well worth the download)