Aquaria.
An absolutely stunning game. The graphics are beautiful, the music is brilliant and the story is... well, hard to explain - killing the bosses made me feel awful, especially the unnecessary ones.
I played most of the way through and then deleted my save files accidentally, and the second time through it felt incredibly strange. I was so drawn into the story the first time that it felt unreal the second, as though I was living my life a second time, knowing what was going to happen - and just to reach a point, rather than to enjoy it. Returning to the first, peaceful areas and Naija's (the main character's) home after I'd been exploring forgotten cathedrals and battling ancient gods filled my with a weird feeling of nostalgia.
Anyway... I don't know if it's the same for anyone else, but Aquaria made me feel incredibly sad while I was enjoying the game at the same time. It's pretty much art, IMO, and definitely a great game.
Also, if no one wants to buy it I can PM them my Humble Indie Bundle key, thus allowing them to download Aquaria for free (along with World of Goo, Gish, Lugaru HD, Penumbra, and Samorost 2). Which reminds me, definitely check out
Samorost 1. It's almost certainly art.