Posing as small-time jewel thief "Donnie Brasco", FBI agent Joseph D. Pistone carried out one of the most audacious sting operations ever in the history of Law Enforcement, working undercover for six years in the 1970s and 80s to infiltrate the East Coast Italian mafia. Pistone worked his way up from an acquaintance of low-rung mafiosi to being a trusted confidant of captain Dominick "Sonny Black" Napolitano.
His testimony led to the eventual conviction of countless top-ranking mafia members.
Pistone's story was later made into a book written by himself (actually readable I might add) titled "Donnie Brasco" and a movie by the same name starring Johnny Depp as Brasco.