ooo let's see.....
well I dunno how non mainstream these are.....depends how wide your mainstream is....but my top films would probably be;
1. Synechdoche, New York
2. Special: Specioprin Hydrochloride
3. Ghost Dog: The Way of the Samurai
4. Mind Game
5. Elephant
6. The Straight Story
7. Factotum (Man of Many Jobs)
8. Wristcutters: a Love Story
9. Waking Life
10. Ong Bak
Sorry, that was 10! I also would've put Edward Scissorhands, Donnie Darko and Being John Malkovich up the top too, but they might be considered fairly mainstream these days.