Of course the idea of the "hidden gem" is personal opinion. Some of these films may have been panned by the critiques or even movie-goers, or like the title suggests - they simply didn't receive much of an audience.
Mine:
[I]Eye Of The Beholder[/i] (w/ Ashley Judd and Ewan McGregor)
[i]The First Power[/i] (w/ Lou Diamond Phillips)
[i]Flatliners[/I] (w/ Julia Roberts, Kevin Bacon and Keifer Sutherland)