While not necessarily a 'hidden' gem, I still feel that Gattaca doesn't get the love it oh, so deserves. Gattaca is a brilliant piece of Sci-Fi, offering everything you want from the genre without being utterly and completely unbelievable or far-fetched. It also has a wonderful score (it does't do anything spectacular but it serves its purpose handsomely), courtesy of Michael Nyman. I definitely recommend this movie to anybody who hasn't seen it!
I think it is one of the most underrated sci-fi films ever made. It explores a lot that has come into the news recently with the exploration of cloning. And the only reason to clone a human is to use them as a slave or as a resource for other people. It is also brilliantly done.
Some people just assume if its scifi it has to bombast and really show a wildly different world. But reality is the future won't look wildly different. Havent seen this movie in a bit, I just remember genetic modification is the big issue and Ethan Hawke's character does something to upset the proletariat.