Music in Twelve Parts
Philip Glass' magnum opus (IMHO).
Probably the greatest work of the 20th century (and probably for much more than that). This is a piece that can change the way you listen to music. Both a theoretical exercise and spiritual journey, its length of over three hours will transport you to a new level of sound and feeling.
Though Philip Glass' avant-garde works are not for the faint of heart, I truly pity those who cannot appreciate the genius of such a powerful work.
Find it here