Some great lists so far. Not sure about the "probably never heard of aspect", as I've heard (or seen) many of the movies from many of the lists, but I'm a movie junkie so there's that. Also the 10-ish list below contains some I'm sure many have heard of, but 1 or 2 obscure ones. Hey, they're just off the top of my head. BUT, none of the ones below have been mentioned yet.
We might need to start breaking these down by decade.
Snatch - Ya like daags?
The Commitments - I'm black and I'm proud.
Fight Club - Mischief. Mayhem. Soap.
It Happened One Night - Lessons on how to hitchhike.
King of Hearts - Which crazy world would you rather live in?
King of Comedy - Robert DeNiro and Jerry Lewis?
It's a Mad, Mad, Mad, Mad World / Rat Race (a remake that's not a remake) - Zany slapstick anyone?
Life is Beautiful - Buon giorno, Principessa! ("No spiders and Visigoths allowed.")
Barry Lyndon, Lolita, or almost anything by Kubrick
Tapeheads - "But first, I'm going to need to you do something for me... on spec."
Repo Man - "let's go do some crimes. Yeah. Let's go get sushi and not pay."