Actually, you might want to take a look at this site to get ideas:
How about Gentleman’s Agreement, about anti-semitism in postwar New York? Or a couple of HBO Films; Something the Lord Made, about Vivien Thomas’ work with Dr Alfred Blalock and Boycott, about the Montgomery bus boycott.
No, Look Who’s Coming To Dinner was the little-known fourth film in the Look Who’s Talking series… 
Neill Blomkamp’s District 9 and Elysium were social justice allegories thinly disguised as SF thrillers.
I’ve used “The Man Who Shot Liberty Valence” with students and gotten lots of interesting discussion. Sure, Jimmy Stewart’s character thought he was a white knight, but so did John Wayne’s. They each thought the other a fool. The newspaperman was a true hero despite being a drunk. Do you stand up to bullies or avoid confrontation? The morality of politics, press, and law, are all considered, as well as “Do the means justify the ends?” Is John Wayne’s relationship with Pompey master/slave, friend, boss/workman? That’s the iceberg’s tip, anyway.
The Visitor. Richard Jenkins, an actor you’ve likely seen in a hundred movies as a background character, was deservedly nominated for an Oscar due to his performance in this movie, which examines illegal immigration and the U.S. in the Patriot Act era. The plot’s a bit heavy-handed, perhaps, but this movie remains very powerful to me.
The East Is Red
Aelita: Queen of Mars
Billy Jack Goes to Washington (“Kunta Kinte would turn over in his grave if he saw you hired out to The Man like this!")
Another good film about anti-Semitism in 1940s America is Crossfire, which was released the same year as Gentleman’s Agreement but is a lot darker.
Some other films that might be worth looking at inlcude A Soldier’s Story and Paths of Glory. However, the latter one might be disqualified under the “no white knights” requirement since it could be argued Kirk Douglas’ character is one even though he fails at his goal.
Django Unchained would be nice.
The upcoming Selma is gonna be an archetype.
The Dark Knight.
The Shawshank Redemption could be a contender. The final “justice” is a bit unorthodox, but it strongly emphasizes the idea that those who wait patiently can be rewarded while those who abuse their power will be brought down. Right soon.