Aaron Wolf's released a jim-dandy Barbershop Quartet arrangement of Nina Paley's song "Copying is Not Theft."
The song is licensed Creative Commons Attribution Share-Alike 3.0 (as is my entire website). This means I have full legal right to do anything I want with it as long as I credit Nina and I license my version the same way. Lots of people have already made tons of variations of the song from jazz to punk rock versions.
Copying Is Not Theft: Barbershop Arrangement pt2
(Thanks, Peter!)