Gog won't let me post links (because I'm too new?), but all can be found easily on youtube:
A few of my favorites:
Samuel Barber, Adagio for Strings (a 20th century piece that, while slow, is incredibly powerful... there's also an a capella choral version: Agnus Dei)
I've always been a huge fan of Beethoven. IMO, his absolutely best work (other than the 9th Symphony, of course) is the 2nd movement of Symphony 7
I'm something of a choral music junkie, and possibly the greatest piece of classical choral work is Mozart's Requiem (featured heavily and to great effect in the wonderful movie Amadeus).
Other great Choral works:
Requiems by Brahms, Faure and Berlioz
As the previous post mentioned, Carmina Burana is awesome.
And there's always the well-known (in some places to the point of being cliché) Handel's Messiah.
A few 20th century things no one's ever heard of: Arvo Part's Miserere (really modern, super creepy), Jean Gilles' Requiem (a 20th century, more oddly upbeat funeral mass... go figure), and Ralph Vaughn Williams' Dona Nobis Pacem. The text of the Dona Nobis Pacem is mostly taken from Walt Whitman's Leaves of Grass (his war poetry).
That's all I've got. :-)
