Ever since the release of Buster Keaton’s 1924 Sherlock Jr, directors have enjoyed themselves with the concept of movies within movies. This is a way for filmmakers to showcase some of their more oddball ideas, prove they're in on the joke and expand their universes in meaningful ways while they're at it.
The fans can't get enough of these either since they heighten the sense of immersion and are never long enough to outstay their welcome.
We've seen some cracking examples over the years, from parodies that made everyone laugh more than most of the fodder that passes for comedy these days, to faux gangster movies you can use to get one over on the pizza delivery man.
Some movies such as Robert Rodriguez and Quentin Tarantino's Grindhouse, and ensemble comedy Tropic Thunder, have used the concept of films within films to great effect, but it's not often that they go on to receive a standalone release.
Machete is a rare example of a movie within a movie that did, and there's a good case for a few other great examples that deserve to follow in its footsteps...