Item 47 is the cryptically-named "one-shot" film included on The Avengers Blu-ray. One-shots, a Marvel creation, are short films related in some way to the feature they're attached to. It will be the third of its kind.
The first two, The Consultant, and A Funny Thing Happened on the Way to Thor's Hammer, came attached to the Blu-Rays of Thor and Captain America: The First Avenger.
Item 47 stars Lizzy Caplan and Jesse Bradford as a neo Bonnie and Clyde, who find a leftover alien gun after the events of The Avengers and use it to go on a robbing spree. Two S.H.I.E.L.D. agents are tasked with stopping the out-of-control couple.
The Avengers and Item 47 come out on Blu-ray September 25. Until then, you can order up some of your favorite Avengers action figures and make internet videos of them being your friends, and check out the new Item 47 poster: