The Gregorian Dies Irae is one of my all-time favorite hymns.  I listen to it every November and use it as a meditation on death and the Final Judgement as well as to remind myself to pray for the Poor Souls in purgatory.
 
The Dies Irae


For the Latin lyrics and the literal English translation:
http://www.franciscan-archive.org/de_celano/opera/diesirae.html

Please don't forget to pray for the Poor Souls in purgatory.  Nowadays with so many people not even believing in purgatory and with so few Catholics left who still pray for them, they are counting on us!

Eternal rest grant unto them Oh Lord, and let perpetual light shine upon them. May their souls and all of the souls of the faithful departed through the mercy of God rest in peace. Amen.