There are TWO scenes during the Avengers end credits. You'll want to stay to catch both of them.
|Even the Avengers think the end-credits scene is funny.|
The second scene comes at the end of the credits. This is a follow up on a joke Iron Man makes a few times in the film. It's kinda like that scene in Spider-Man 2 where Spidey rides the elevator... hilariously awkward.