I tend to be the friend who tells people "oh, I have to make you watch this movie!" Here's the thread for people to explain what movies they make their friends watch and why:
Sideways--not really an obscure one, but if someone hasn't seen it I always recommend it. Not what you expect for an Oscar-nominee, too many hijinks!
Wonder Boys--Michael Douglas, Robert Downey Jr, Frances McDormand (the Fargo lady,) Toby McGuire, Katie Holmes, Rip Torn. Does that list convince you? If it doesn't, perhaps seeing Michael Douglas in a pink bathrobe toking up will. Or James Brown lookalikes leaving butt-prints in car hoods? Tuba-playing transvestites? Dead dogs? Bueller? Bueller?
Suicide Kings--Dark comedy. If you like Christopher Walken, it's a must. If you need one, it's a good cautionary tale about kidnapping a mob boss.

Boondock Saints--I can't watch this movie without dropping a million f-bombs afterwards. But the accents are completely worth it. Plus, Willem Dafoe is scary and funny.
The Birdcage--Robin Williams and Nathan Lane as gay cabaret owners who have to play it straight to meet their son's fiancees family. Plus it doesn't get any funnier than Hank Azaria in Daisy Dukes singing Gloria Estefan, talking about his Guatamaleness.
Drowning Mona--kinda kooky comedy. Bette Midler as a nagging housewife who noone really mourns for after she's gone. Soundtrack features a lot of Three Dog Night, and not only do you see Jamie Lee Curtis in a mullet, there are Yugos as far as the eye can see.
Chasing Amy--Kevin Smith, how I adore him. Priceless movie, if only for the horrible horrible things that come out of Jason Lee's mouth. Like "I'm telling you that chick is probably a bigger germ farm than that monkey in Outbreak."
Thank You for Smoking--a more recent one. I almost peed myself laughing. Only for people who appreciate irony.
Those are just a few (I left out all my oldies but goodies, like the Hitchcock ones.... people need to see that stuff!)
What about you? What movies do you recommend to everyone you know?