Hostile said:
The Frisco Kid.
If you haven't seen it, you're missing out. Hilarious.
That's definitely a sleeper I gotta go with. I love that movie too! It's amazing that even with a big name like Harrison Ford, I'd still consider it a "sleeper" flick.
I think the choice of Arlington Road is also a good one. I love that movie! Jeff Bridges' earlier movie, Blown Away, got so much attention that I thought going into this one that it was some sort of sequel, but it was much better, and I really dug the ending.
Trying to think sleeper (cause I don't consider half the movies listed as true sleepers):
187.... Samuel L. Jackson, before his overdramatic acting style was that well known, novel approach to the cliche teacher inspires the ghetto kids story
Punch Drunk Love.... yeah, I know big name actor and director, but really, when is the last time you heard anything about this movie, this was released when Adam Sandler was pulling in $200 mill per movie and it barely made a noise even though it was a highly entertaining flick
K-PAX.... going with the Jeff Bridges theme, this is really just a rip-off of a movie he made 20 years earlier called Starman, but still an entertaining flick
One Hour Photo.... i guess Robin Williams isn't as popular as he used to be, but I think he makes a great psycho, unlike one of his other popular psycho roles in Insomnia, this one didn't get near as much attention
....just a quick brainstorm, I could probably name several others.