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:

Eye Of The Beholder (w/ Ashley Judd and Ewan McGregor)
The First Power (w/ Lou Diamond Phillips)
Flatliners (w/ Julia Roberts, Kevin Bacon and Keifer Sutherland)