Brotherhood of War
The best war movie I have seen, I'm lucky to have randomly rented this one.
http://m.imdb.com/title/tt0386064/
Splice
Great gene-splicing sci-fi that I begrudgingly watched at my partner's request, but ended up loving.
http://m.imdb.com/title/tt1017460/
OldBoy
Not the shitty remake. This probably doesn't count as under appreciated, but it's hard for me to tell as no-one I know personally has even heard of it. Do yourself a favour and watch this madness. Also, the octopus is real.
http://m.imdb.com/title/tt0364569/
Catfish
An excellent documentary/mystery thriller. The show it spawned is average, but the movie is great.
http://m.imdb.com/title/tt1584016/?ref=m_nv_sr_2
Orgazmo
Probably Matt Stone and Trey Parker's best work. This is comedy gold.
http://m.imdb.com/title/tt0124819/