"Lux Aurumque" by Eric Whitacre Singers
A favorite a capella (no instruments, only vocals) track.
We sang a TTBB arrangement of this when I was singing with the Columbus Gay Men's Chorus. It was one of my most favorite songs that we ever performed. And singing it under the direction of David Monseur made it all that much better.
There's a TED talk about it - the song, not me.
Eric Whitacre: A choir as big as the Internet (ted.com)
