Donnie Brasco (1997)

Two great performances by Jonny Depp (as Donnie Brasco) and Al Pacino as Lefty, a gang member.  The story is about Donnie Brasco, an under cover FBI agent, infiltrating Mafia gangs in New York City (with some of the action also in Florida).  The operation which was meant to go for 6 months, continued for 6 years and ended when Donnie was being ordered to murder a gang member.  The events this story is based on took place from 1976 to 1982.

An excellent movie with great reviews.