Sammy "The Bull" Gravano

Sammy Gravano is/was an American mobster and former underboss of the Gambino crime family. He was an instrumental figure during John Gotti's trial as his information helped bring him to justice. This is highly unusual as people in his position in a crime family do not rat or provide information.

There's a great movie about his life available on YouTube.

The movie is called "Witness to the Mob" and it comes in two parts on YouTube.