If for nothing else, the line “Back off man. I’m a scientist.”
Recently I would add:
Napolean Dynamite
A little older:
Monty Python’s Life of Bryan (Actually, the entire Monty Python collection would do).
Better Off Dead “I want my two dollars”
Princess Bride - Most geeks don’t admit to liking it, but at certain moments, you’ll catch me . . . Uh, I mean, them . . . quoting it.
Hackers!
Hack the planet!
Aieee! Showgirls??!! As a female geek, I protest. That is not a geek movie. It is just a guy movie. And not all geeks are guys.
Office Space and The Wall are great choices, though.
No-no-no. Willy lacks geek cred.
And at least 2 Anime qualify besides GITS–Voltron (“FORM BLAZING SWORD!” “DYNATHERMS CONNECTED! INFRACELLS ARE UP! MEGATHRUSTERS ARE GO!”).
And every Geek worth his salt can sing the English themesong from Star Blazers.
Most of my geeky friends are fond of Fight Club and Pulp Fiction, both of which have lots of quotable lines.
Well, in addition to many that have alrready been mentioned, there’s plenty of quotes from The Large Rouge One bouncing around my particular batch of geeks as well.
“Go to Red Alert!”
“Are you sure, Sir? It does mean changing the bulb.”
As long as we are tearing down movies, I offered and then rejected Labyrinth in post six.
Throw me in for a vote in favor of willow for the “when you are drunk, I am in charge!”
“The I stole the babaaay!” is a regular one as well.
I have also seen alot of mileage from “Of course not, you’re a boy” from the dark crystal.
Since Ghostbusters and Tron have already been nominated, I’ll toss out:
Wargames
The first real “hacker” movie for the masses, plus Joshua’s climactic revelation via tic-tac-toe, along with the immortal line, “Shall we play a game?” 
My measure of geekness is would Dean from Heart of the City go all fanboy over it? So far I’ve seen him do:
Star Wars
Star Trek
Lord of the Rings
Close Encounters of the Third Kind (well, UFOs in general)
*Pirates of the Caribbean: The Curse of the Black Pearl
Six String Samuri *(If that’s not quotable, nothing is.)
Real Genius
U.H.F.
And a few books, too:
Lord of the Rings
Ender’s Game
Snow Crash
Anything by Terry Pratchett
Every one. I’ve seen every God Damn one. I think I need to rethink my lifestyle.
From the Kurt Russell collection:
Big Trouble in Little China
The Thing
Tango & Cash
Others:
Time Bandits
Spaceballs
Young Frankenstein
Real Genius
Top Secret
Though I think there might be two hierarchies of Geekdom.
Sci-Fiers:
Star Trek
Star Wars
etc.
Fantasiers:
Labyrinth
Willow
etc.
With some shared films:
Blade Runner
Princess Bride
etc.
Perhaps? Or am I only half geek (fantasier)?
Come on people! Clearly you are not hanging around the right geeks (and clearly you cannot chose the wine in front of me). Peck is a favorite insult amongst some of my friends (Well, that was really stupid, Peck! Peck peck peck peck peck peck!)
Labyrinth:
*'elo!
Did you say “hello”?
No, I said “'elo,” but that’s close enough!*
Or, even better, and much more applicable to random situations:
You have no power over me!
(there are probably others, but I’ve been out of geek company of late)
Neverending story:
*They used to be such good, strong hands.
Falcor!!! Atreyu!!!*
We don’t? Dude, that’s my favorite movie, and I’ve never denied it. And I’ve never heard any of my accquaintances (nerds all of them) claim to dislike it.
Also, I’m not sure that Dune belongs on this list. The novel, most definitely, but I think that the screen adaptations were only significant as reflections of the novel.
Now, then, on the basis of referencability, I’d have to say:
Monty Python and the Holy Grail (am I the first to nominate this one?).
The Princess Bride.
Episodes IV, V, and VI of Star Wars (the prequels’ status remains to be decided by history).
At least some Star Trek… Certainly The Wrath of Khan, probably The Voyage Home, perhaps others.
The first two Indiana Jones movies-- I’m unsure about the third one.
Real Genius.
Weird Science.
Ghostbusters.
Alien and Aliens, but nothing else from that series.
The Day the Earth stood Still– Nobody remembers anything but “Klatuu Baratta Nikto”, but that’s enough.
Army of Darkness, but not Evil Dead 1 or 2. They’re not really connected, and the first two were straight gore-horror.
Forbidden Planet. Have you ever seen Robby’s IMDB entry? That’s enough reference for me.
Young Frankenstein. Possibly some other Mel Brooks movies, but that one’s a must.
Men in Black, at least the original. I don’t know the sequel.
At least the first Matrix movie, and possibly all three. The third one wasn’t nearly as bad as everyone says.
Peter Jackson’s Lord of the Rings.
That’s the beauty of being a geek, you can multi-class easily.
I would agree with almost all of the movies on here , but Hackers? No, not ever, not even in the “so bad it has got to be good” category. Worse than The Matrix sequels.
Big thirds on 2001 and Buckaroo Banzai!
“Open the pod bay doors, Hal”
“Evil. Pure and simple from the eighth dimension!”
To add a book/author:
Do Robots Dream Of Electric Sheep? -Phillip K. Dick, almost anything else by him.
Nothing personal, Noelq. I hate that movie with a passion, though. It has more to do with skateboards than computers.
Maybe it’s just me - and the geeks I hang out with - but I’d definitely include Dr. Strangelove . And most Mel Brooks movies.
I of course agree with all Monty Python, The Day the Earth Stood Still , and MST3K, and would like to add Invasion of the Body Snatchers (a personal favorite?).
Since the thread hasn’t branched out to series much, I guess The Simpsons are still out… but obviously a must.