Chopin's Piano Concerto No. 1 In E Minor, Second Movement
I think this is possibly the most beautiful piece of music in the whole world. My mother used to play it on the piano when I was a very small girl, after I had gone to bed. I would lie in the darkness and feel very happy and sad at the same time.
Now I have it on CD, and sometimes when I have the mean reds and can't shake them, I listen to it over and over again. It's the only thing that helps.
This is a decent performance I found on YouTube: http://www.youtube.com/watch?v=Mg24YsN0iuU