Err, Black Hawk Down is not underrated. It is well rated and appropriately so. Ditto Heat. Ditto Ronin. Ditto 12 Monkeys. Ditto The Game and Kiss Kiss Bang Bang, even if they are less known.
World's Fastest Indian is overrated. I lurv Sir Anthony and it's all well done and everything but it is essentially just an old man character muddling around for a couple hours (overcoming the odds schtick doesn't do much for me). Not exactly entertaining and I don't mind slow drama.
So yeah, I reckon a lot of folks are missing the point of the topic. Not popular does not equal underrated. Indeed, popular often equals overrated. Just look at some of the shite on imdb's top 100: Lord of the Rings trilogy in the top 10? Laughable.
The Mosquito Coast is one of the few mentioned recently which is truly underrated. Another which comes to mind (indubitably due to like setting) is The Emerald Forest.
I propose axing any title from the list which is rated 7 or more at imdb.