Many Marvel films include an extra scene after the credits roll and The Avengers is no exception. Actually the movie contains two bonus scenes! One will occur after the main end credits and a second scene will be included at the very end of the credits.
The final scene has something to do with what Tony Stark says at the end of the movie itself.
The Joss Whedon action adventure has already earned over $218.2 million internationally, but this scene has not been included in those screenings.