What are some examples of good films that were not seen by many due to horrible marketing?

Examples I can think of are Idiocracy and Slither - so any genre really - Sure I'm not alone in loving when I find a movie whose trailer did it total injustice, or I've never heard of, only to be blown away. I'm writing a paper that I have to present with clips. Plus it'd be cool to find a gem to watch.

What movies and why?