I've noticed how they post these on the '90s board about how it relates to contemporary values, but for people seeking a trip deep down memory lane, certain movies film during the actual era can reall get you acquainted to what really went on during that time. For instance, a film made during the '30s that is a good earmark about the Depression or Nazi Germany? Or '40s WWII films (not actual historic footage), '50s films (for instance Blackboard Jungle is a good approximation of the high school era as opposed to those rose-tinted American Graffiti and Greases), and '60s movies that show the effects of the changing world. Also take into account the fashions/goods/services used in each particular era.

I think "The Grapes of Wrath" has to be a film talked about here.  It was made at the tail end of The Depression and I'm sure accurately depicted what 1000's of families in that social class may have gone through during The Depression.  It's been maybe 15 years since I saw the film and originally watched it about 25 years ago because I'm a huge Henry Fonda fan.

Another but it probably doesn't fit into your criteria because it was filmed 20 or so years after the events that was suppose to take place was "The Great Gatsby".  I've only seen it one time, caught it on tv years ago, I've seen the more modern version a few more times than that, but it shows the a pretty good picture of the Roaring 20's or at least that social class.

Although I've never seen it I hear "Rebel Without A Cause" is a social relevent picture of it's time.  Same goes for "Easy Rider" for the 1960's and what the movie showed and represented.

There were literally dozens of films made during World War II about the war and the ones I have watched I always had the impression these were glamorized a bit.  Maybe that was due to keep the moral up of the people waiting home for loved ones to return, I'm sure that was a huge part of it.  The one I always heard portrayed WWII accurately though again was made a few years after the war and that was "To Hell and Back" the film about Audie Murphy.  It isn't carnage filled like "Saving Private Ryan" but you supposedly get the idea of what a military unit went through at that time.

The other night just watched for the third of fourth time, "The Diary of Anne Frank".  Again a film made after WWII probably doesn't fit the criteria you're looking for.  I've never read the book, I'm not sure why, I love history, but a lot of people that have read the book really didn't like the film.  But watching the film, getting into the characters, trying to fit in their shoes that how scared they must have been knowing at any time the Nazi's could discover them.  Living their lives in those cramped quarters, their entire world only that 20 x 20 room or whatever it was.  Unimaginable.  Then being discovered and taken away to the concentration camps, again the horrors millions of people went through, again I use the word, unimaginable. 

Anyway I love watching these old films on TCM.  Some movies showing some great social significance as every decade as their share of films like that, and others are pure fluff which isn't that what movie audiences are after to begin with?