After hours (starring Griffin Dunne) surprisingly didn't get the credit it deserved, so that for me is a very under rated film.
Sat and watched se7en last night since there was nothing else worth watching. Even when you know the twist it really is a cracking film. That along with fallen are two of my favourites from the 90's
Carlito’s Way
The magnificent Donnie Darko, if that can be considered underrated...or a Uruguayan film about a sock factory called Whisky which was loaded with dark humour...Underrated by whom?
Hot Tub Time Machine. I don't know if it's underrated but I remember hearing the name of it thinking it would be p1sh but it is pretty decent.