Started in 1994, The Publius Enigma is a puzzle that uses Pink Floyd's 1994 album "The Division Bell" as it's source.
Prizes for the one who solves the enigma have been described as "unimaginable", and the one will receive "worldwide celebration."
Clues come from the music and arrangement of The Division Bell as well as it's artwork. They also come from posts on the Pink Floyd message boards by a person/people calling themselves "Publius", as well as later as "Genesis."
A comprehensive list of the clues given out and found can be found here.
The Wikipedia article on the Publius Enigma can be found here as well.