Whitacre creates such gorgeous, unexpected harmonies in his music. One of the best examples I know and one of my favorites of his a cappella choral works is Sleep:
Performers in this recording include the Vocal Essence Chorus & Ensemble Singers, the St. Olaf Choir, and the Minnesota High School Honors Choir, directed by the composer, himself.
Interesting note: Whitacre originally wrote this song using Robert Frost's "Stopping by Woods" as the text, but couldn't get the poet's estate to release the use of the poem. So he asked his friend, poet Charles Anthony Silvestri to set words to the music he had already written. The first few lines of the poem are as follows:
The evening hangs beneath the moon,
A silver thread on darkened dune.
With closing eyes and resting head
I know that sleep is coming soon.
Read the rest here.