This is not an underappreciated movie per se.  I just feel that more people should watch it.

https://www.youtube.com/watch?v=dVRnG1MddAM

The Sea Inside

It only partially quaifies since it pretty much got all the critical acclaim it righfully deseved it even made numerous top 10 list for that year.  Despite that I still think its somewhat underrated and underappreciated.

I wasn't sure if you were joking or not. I watched JCFM. Not bad TBH. It was Battleship that threw me off, because it's one of those movies that hasn't had good word of mouth among my friends, and at a glance, it wasn't feeling it. I loved Kung Pow! though, so what the f*** do I know?

Battle Royale. If I remember right, it wasn't a hit with reviewers. Though I see it as one of the best movies I've seen.

Cabin in the Woods might fit that description too.

Battle Royal was great, but being that it is a Japanese film with subtitles it is bound to not get acclaim or recognition.

If we include foreign films one of the best ones I have seen is Curse of the Golden Flower. It is a cenematic masterpeice, imho.

Gattaca (1997) - Ethan Hawke, Jude Law, Uma Thurman

Wondeful movie dealing with a not too distant future where every child is genetically engineered to be "perfect". The movie follows Vincent (Ethan Hawke) who was born the old fashioned way and is thus considered inferior and restrained to menials jobs.

In order to achieve his dream of being an astronaut, he takes on the identity of Gerome (Jude Law), who was destined to a brilliant future until he got disabled in a car accident. Everything is going according to plan until a murder occurs at his office and one of Vincent's eyelash is recovered as the sweep the place for evidence.

The movie is a great look at prejudice, perseverance, potential, raises interesting questions about genetics. The relationships between the characters are well developed (especially between Gerome and Vincent but there's also the relationship between Vincent and his brother too) and you feel really invested in Vincent's dream and want him to succeed.

SLC Punk. Best Mathew Lillard performance ever IMHO of course.

I loved Stay!  I saw it in the theater and own it on DVD.  I just watched it again recently.

Am I the only one who liked "In the Heart of the Sea"?  I remember it bombed at the box office, and wasn't well received by critics either.  I saw it in the theater though and enjoyed it so much that I regretted not paying extra for IMAX.  Warner Bros shot themselves in the foot with it's release.  I remember reading that the movie was ready to release in March of 2015, but they decided to hold it until December to try and get some Oscar season buzz.  The huge problem there was that they released it 1 week before "Star Wars: The Force Awakens".  That means it was going to get kicked out of all IMAX screens and even most regular screens within a week.  I remember when I saw "Force Awakens" the week after "In the Heart of the Sea", my local AMC theater was devoting 17 of its 20 screens to Star Wars.  So, Warner Bros didn't really put "In The Heart of the Sea" in a position to have any success in the first place.  Still, I've watched it more than once and think it is highly underappreciated.

https://youtu.be/K-H35Mpj4uk

Plenty of classics and well-known movies mentioned in this thread.

Now I have movies for the entire year :D