The question is, do you mean movies that weren't seen in their day, or movies that aren't seen now? Because if it's just movies that younger people haven't seen...
Lawrence of Arabia - I know most of you probably have heard of it, but I don't think there are a lot of people these days clamoring to see a three hour movie about battles in the middle east during World War 1. That said, it's one of the most beautifully shot movies ever made. It is a piece of art.
Amarcord - Great Fillini film that explores his childhood in Fascist swinging Italy. In my opinion, a better film than the similarly themed 8 1/2.
Sneakers - Great hacker/heist film from the early 90's. One of those movies that always just makes me feel warm inside, and I'm a cold hearted bastard.
Chinatown - Classic Noir at its best. Amazing movie.
Lonely are the Brave - Great modern western (modern to the 60's) - Kirk Douglas is a cowboy who can't fit into the modern world.
Wings of Desire - German film from the 80's, very arty, very cerebral, explores life in West Berlin through the eyes of angels. Uses color as metaphor.
As to movies that are coming out now that people haven't seen...
The Edge of Heaven - Modern film from Germany about the Turkish community, three stories happening to connected people at the same time, all about the fact that they never intersect, but they always come within a hair of it.
Anything by Pedro Almodovar - I started with "Talk To Her", but you can really go anywhere. Other greats from him, "All About My Mother," "Volver," "Tie Me Up, Tie Me Down!" "Women on the Verge of a Nervous Breakdown."
Assisted Living - Really small movie from a few years back, explores aging in a nursing home from the perspective of a janitor working there and his budding relationship with an alzheimer's patient.
Six String Samurai - Ridiculously fun movie about a post-apocolyptic world where the soviet union won the cold war. Las Vegas (now called "Lost Vegas") is the last bastion of capitalism where Elvis is king. Elvis has just died and Buddy Holly is making his way through the desert with his six-string and his Katanna to claim the throne, all the while on the run from Death and his acid rock band.