What are some of the most memorable lines of dialogue from a game you've played?
For me it's gotta be, "Go for the eyes Boo, go for the eyes! Yeeaargh!"

Bas Rutten from GTA 4:

"When you're done, you will be able to go up to a woman and whip it out, and ask what are we gonna do about this?"

"Your princess is in another castle."
"Dodongo dislikes smoke."
"It's dangerous to go alone! Take this."

I still say "Squeaky wheel gets the kick!" and then people say "You mean the oil?"

I'm also a fan of "All your base are belong to us."

"Another visitor! Stay awhile! Stay FOREVER!"

Both Portal games are full of fantastic lines, actually. Way too many to list here.

Yggdra Union: "Justice lies with the holy sword!"
"Come near me and I'll kill you!"

The Mechwarrior games: "All systems: Nominal".

Warhammer 40,000: Dawn of War has a lot of good quotes, but the Imperial Guard stand out with some of the best quotes:

"Ready to unleash eleven barrels of Hell!" - Baneblade
Commissar:
"Fear ensures loyalty! Where do you need me?"
"If you will not serve in combat, then you will serve on the firing line!"

And last but not least, from General Sturnn, "To each of us falls a task. And all the Emperor requires of us Guardsmen is that we stand in line, and we die fighting. It is what we do best: we die standing."

And this one is from Final Liberation. (Yes, I want this to go to GOG.com) Commissar Holt is badass on this one.
http://www.youtube.com/watch?v=LOi3CzZjV0M

"Choose your next words with exceptional care!"

HEY! IT'S THAT DOG!

Resident Evil 4

Great thread. :D

Oh and....

"WHAT!? You don't sell Postal!?" from of course, Postal.

"Did you see that, Zach? Clear as a crisp spring morning. FK...in the coffee!"
-Francis York Morgan, Deadly Premonition

"You must gather your party before venturing forth"

My vision is augmented.

That's the second biggest monkey head I've ever seen.

War. War never changes.