I'm not sure if documentaries are out-of-bounds for this discussion. I suppose any movies can be considered...and if this is the case, I'd point to Al Gore's film, An Inconvenient Truth as a film that helped to bring wide-spread attention to ecological issues.

These issues were not exactly secret before the film, but the movie provided an added traction and momentum to a growing "ecological consciousness" movement. 

I have to say that Mississippi Burning, A Time To Kill, and To Kill a Mockingbird have all been influential in society. All of the movies have depicted the problematic and oppressive nature of racism in the South. These films helped to open the eyes of those who may have been ignorant to the oppression African Americans were facing at the time.

One film that did this was The Birth of a Nationfrom 1915.  This was a film that portrayed the KKK as defenders of right in the South during Reconstruction.  It portrayed black ex-slaves as animalistic predators.  This helped to perpetuate racist attitudes towards African Americans in the United States.