How could no one of mentioned Steven Speilberg's epic "Schindler's List"?
It's the greatest movie ever made.

fix-the-spade:
Its been briefly mentioned above but:
HEAT

its a long movie, but after the first 10 minutes I don't think I even blinked until the end, never mind look away.

Oh hell yes. Brilliant movie.

ingsoc:
Grosse Point Blank, Threesome, Dark City, Event Horizon, Eight Heads in a Duffle Bag, They Live, Pi (mathematical symbol), Trainspotting and Gattaca (I am always surprised that not a lot of people know about this flick) to name a few.

I'm surprised how few people know Gattaca, too, especially since it's a fantastic film. I'd also throw in the other John Cussack classic: High-Fidelity.

Action movie: Boondock Saints

I'm with Khell on Donnie Darko as a royal mind-fuck

Comedies: Liar, Liar; Clerks; Mallrats; Chasing Amy (closer to romantic comedy, though); 10 Things I Hate About You

If you can stand anime there were a bunch of good Miazaki films. Castle in the Sky, Kiki's Delivery Service, My Neighbor Totoro, Porco Rosso.

Almightyjoe:
Do they need to be pre 2000? aww

i'm gonna cheat anyway, The Prestige

Hugh Jackman, magic, Murder, Tesla, and...

I WARN YE! TO READ THIS SPOILER IS TO NEVER BE ABLE TO ENJOY THE MASTERPIECE FOR WHAT IT TRULY IS

Spoiler: Click to View

it doesn't get much cooler than that

LOL, I hope people have seen the movie!

A few brilliant films such as Forrest Gump, Waynes World 1 & 2, Austin Powers etc. However Oldboy was released in 2003 and that's insane too.

Fight Club. (Brad Pitt, Edward Norton)

101 movies all men must see before they die:
most are well known and after 2000, but a few fit the criteria. Although i don't think there's more then 5 in the list that doesn't have some hot-shot actor in it.

http://www.justaguything.com/100-great-movies-every-guy-should-see/

Film4's 50 movies to see before you die:
this one's better although the no.1 spot is 'Apocalypse Now' there are a couple of good entries and it really takes into account foreign film making

http://www.channel4.com/film/reviews/feature.jsp?V=3&SV=2&id=161521

Bad Taste and Escape From New York :-D They are some of my all time favourites

The Apartment - The oldest film ive seen, loved it, depressing subject matter has never been done in such a brilliantly cheerfull way

Grave Of The Fireflies - A studio ghibli minus childfreindliness + unrelenting sadness punctuated with short lived optimism and genius

Cowboy Bebop the movie - Should have been crap -a anime tie-in film aside from the normal plot... but... well you can guess :)

I am shocked and ever so slightly appalled that only one person so far has said Goodfellas. By far and away my favourite film of all time, a proper gangster masterpiece.

A few more:
Aliens
Friday The 13th
Nightmare On Elm Street
Pulp Fiction
Reservoir Dogs
The Matrix
Lock Stock And Two Smoking Barrels
The 3 original Star Wars
Fight Club
Se7en
Apocalypse Now
Platoon
Saving Private Ryan
A Clockwork Orange
Taxi Driver
Raging Bull
Leon
and the first 3 Indiana Jones films.

Depends on what you like, really.
War:
-Platoon
-Full Metal Jacket
-Apocalypse Now
-Patton
Comedy:
-A Midsummer Night's Sex Comedy
-Mighty Aphrodite
-Love and Death
(I know those are all Woody Allen movies, but I do like the guy's work very much)
Drama
-The Lion in Winter (Peter O'Toole/Catharine Hepburn version)
-On the Waterfront
-Room with a View
Science Fiction
-The Fifth Element
-Blade Runner
-Ghost in the Shell
-2001: A Space Odyssey
-Planet of the Apes

Of those, I'd pick Platoon, Blade Runner, and Mighty Aphrodite as the best. Enjoy!

I'm betting at this point someone has mentioned them but just in case, every movie made by Kevin Smith and every Mel Brooks movie. 2 shining examples of comedic writer/directors.

The Fly. The Fly to Alien is what His Dark Materials is to Harry Potter. Its better, but it came out a little too late, and was left behind.

Haven't seen these yet:

Godzilla (the American one that came out in '98)
A Bug's Life (Pixar is brilliant)
Toy Story (again, Pixar)
Jurassic Park (suprising that this wasn't said)
The Lost World: Jurassic Park (opinions vary)

That's all I can think of for now.

Dude A Clockwork Orange is a freaking diffucult movie to piece together, great movie though.

Also twelve Angry Men was a GREAT movie

Wow, I feel like I've come to the party late.

Godard's Breathless, not the remake with Richard Gere. It will help you understand film so much more.
Man Bites Dog, it's a French faux documentary. Top notch and one of the most disturbingly hilarious films I have ever seen.
What's New Pussycat, Woody Allen's first big script that got made. Peter O'Toole is fantastic.
Speaking of Woody Allen, check out Take the Money and Run. Woody Allen is at his best when he is absurd.
Dr. Strangelove has been mentioned quite a bit on here, but it's only because it's possibly the best film ever made.
Jim Jarmusch's Night on Earth is great. A fantastic cast and downright funny.
North By Northwest is leaps and bounds my favorite Hitchcock flick. Cary Grant can do no wrong.
Last but not least...The Big Sleep. Screenplay written by Faulkner, based on a Raymond Chandler book, directed by Howard Hawks and...wait for it...starring Humphrey Bogart. You couldn't beat that with a stick.

There are so many more where they came from. Check out french films from the late fifties and early sixties. That's when the foundation for modern film was laid. Truly mind blowing.