Wolfgang Amadeus Mozart's Requiem in D minor (K. 626) is considered by many to be one of the greatest works of classical music. It was composed in 1791, supposedly as Mozart lay on his deathbed, but Mozart was unable to complete the work before his death.
Popular legend says that an unknown messenger commissioned the Requiem from Mozart, but it's now believed by many scholars that the patron was Franz Anton Leitgeb, a Vienna music lover who gave concerts at his home. The Requiem was commissioned for Leitgeb's late wife, but Mozart was fond of telling his wife that he was composing the Requiem for himself (he also enjoyed imagining that he was being poisoned).
At his death, Mozart had finished only two sections of the Requiem, but the notes he left allowed two of his students to complete it.
Most people know of the Requiem because of the film "Amadeus", which used large portions of the piece to masterful effect. Because of its prominence in the movie, the Requiem may now be Mozart's most recognizable work.