For more than thirty years, a mysterious masked prisoner known only as "Eustache Dauger" was incarcerated under the regime of Louis XIV—first at the king's fortress in Pigernol, and later at the Bastille. (The Straight Dope's Cecil Adams runs through the story here.) He died in 1703, and by most accounts his secret died with him—but public speculation about his identity only increased in the years following his death. Voltaire, who was something of a prankster, created the modern legend more than a half-century later by describing the mask as iron (it was more likely made of black velvet) and famously proposing that the prisoner was King Louis XIV's unacknowledged brother, a theme that carries over into Alexandre Dumas' Three Musketeers novel The Man in the Iron Mask (1850) and subsequent film adaptations.
io9's Esther Inglis-Arkell recently endorsed the most plausible theory to date, which is that the Man in the Iron Mask was actually a disgraced French general by the name of Vivien de Bulonde, who abandoned the Siege of Cuneo (and his equipment, and his wounded soldiers) in 1691 in anticipation of an enemy's nonexistent reinforcements. This theory was advanced by military cryptologist Etienne Bazieres (1846-1931), who decoded a previously indecipherable letter from Louis XIV reading as follows:
His Majesty knows better than any other person the consequences of [Bulonde's] act, and he is also aware of how deeply our failure to take [Cuneo] will prejudice our cause, a failure which must be repaired during the winter. His Majesty desires that you immediately arrest General Bulonde and cause him to be conducted to the fortress of Pignerole, where he will be locked in a cell under guard at night, and permitted to walk the battlements during the day with a...
And that's as far as Bazieres could directly decode the passage—the word that appears at the end of that sentence is not one that appears in Louis XIV's other encoded letters. Curiously, "masque" would work; it wasn't a word Louis XIV used elsewhere. And both Bulonde and the masked man were imprisoned at Pignerole. And the masked man was arrested shortly after the Siege of Cuneo, following transmission of the letter.
Could it have been someone else? Possibly—but unless we exhume the masked prisoner's mysterious grave (marked only "Mathioly") and discover DNA evidence that gives us a better lead, the best working theory is that he was, in fact, Vivien de Bulonde.