Eric Whitacre has created a "virtual choir" using volunteers from YouTube. The choir members applied via audition videos like this one, and Whitacre then posted a "conductor track" for his choir to sing along to (the piece is "Lux Aurumque" by Whitacre, written in 2000). By having the choir sing along at home, with a loud "BEEP" sound at the beginning of the video (used for synchronization), Whitacre could then assemble the video and audio of his 200 choir members to create one master performance. It's astonishing, though Whitacre's conducting looks a little goofy at times. Check it out:
See also: an earlier performance of the song "Sleep" by Whitacre's first virtual choir.