The other day I made passing reference to the ubiquity of the plainchant tune Dies Irae.
Ever helpful, the BBC has put together a program discussing the piece on the Radio 3 series Twenty Minutes. It's called Dies Irae: Day of Wrath and offers a decent 1/3 hour overview of a few ways the piece has been used over the centuries. (It also reminded me how thrilling Berlioz's Symphonie Fantastique can be.)
The piece should be up on iPlayer soon at this page. The Wikipedia page about Dies Irae has a list of some uses of the tune, too.