40

I thought it was widely known what an incredible song "40" was, by U2. But I've learned it ain't...
SO--you can read a little background here... then go spend the 99c at itunes, (or 1.29 if you've switched over):

This was the last song written for the album. They had already used an extra week of studio time and needed one more song in a hurry, so Bono opened a bible, read from Psalm 40 of Psalms of David, and they put it to music.

During a live performance on April 29, 1987, Bono joked: "We spent ten minutes writing this song, ten minutes recording it, ten minutes mixing it, ten minutes playing it back, and that's got nothing to do with why it's called '40'."