Today is Good Friday, one of the most important days of the year for Christians. If I had to choose music which represents this day for me, it would be a classical piece: Lacrimosa. It's part of Mozart's Requiem. I generally don't appreciate Mozart's music much, except for this Requiem. Lacrimosa was the last piece of music Mozart worked on. He died after writing the first part. Lacrimosa (along with the rest of the Requiem) was finished by one of his students.
If you might think you don't know this classical piece. You're probably wrong. It's a very often used. You could have heard it as a sample in Sweetbox's "Everything's gonna be alright" or (very well integrated) in Evanescence's "Lacrymosa".
Lacrimosa is Latin. It means "I weep" and it's part of the Dies Irae, the "day of wrath". The reason it fit's Good Friday so well is for two reasons. On Good Friday Christians remember that Jesus died for their sins. So on Good Friday "I weep" because of my sins (reason one) and the fact that it was necessary for Christ to undergo that much suffering (reason two).
If you never heard it: you should. Especially today.