Photopia made me cry.
That's not something I say often. I don't think any other work of art has ever affected me to the extent that Photopia has.
I say "work of art" there partly because that's what Photopia is, a magnificent work of art, but mostly because I hesitate to call it a game. Photopia is very, very linear. It has very simple puzzles. It's barely interactive at all.
And yet it works. Photopia could be a short story, but it would lose most of its impact. It's difficult to explain why that is without ruining the game. The key to Photopia's success is the interactions between the player and the main character (who, interestingly enough, is never actually playable).
Photopia takes the term "interactive fiction" to a new level, because that's really what it is. It's a short story with interactive elements. It's unlike anything else I've ever played, and it's absolutely genius.
I'll be honest; I'm finding this very hard to write, because Photopia does so many things well, but discussing any of that would spoil the game, and, frankly, spoiling Photopia would completely ruin it. The emotional impact of this game is ridiculously effective. It's beyond words.
Note: Photopia requires a Glulx interpreter to play. The game comes bundled with an interpreter for Windows, but to play Photopia on a Mac or Linux box, you need to download an additional interpreter -- links above.
In addition, Adam Cadre put together an FAQ that explains some of the influences behind the game. It's an interesting read once you finish the game, and it's also linked above.