So much has already been written about Fumito Ueda's Ico that it's hard to know what else to add. If you haven't played the game, it's a pretty fundamental piece of work. Sony is doing an HD re-release for the PS3 as a buildup to Ueda's new, highly anticipated The Last Guardian, but you can generally pick up a used copy for the PS2 online.
What I found the most striking about this game was the incredible restraint that it had in terms of music, 'editing' or other mediation- letting the atmosphere and sound design of the world speak large- the situations of the game are incredibly emotionally evocative in ways that games and movies so rarely are, even when they hammer you in the face with fast cuts and John Williams' scores.
The game has a dreamlike, simplistic plot; you play a boy born with horns- an occurrence viewed as an omen in your village. You're taken to a crypt in a vast, deserted castle, and sealed into a tomb as an offering. You manage to escape, and find another inhabitant in the castle- a luminous girl who you are unable to communicate with, who is being constantly threatened by shadowy, amorphous figures. That's another powerful omission in this game- the lack of exposition adds to the stark feeling of the gameplay. I can't say enough about the feeling of immediacy that this creates as you make your way through the game.