"Ah jeez, I'm runnin' late..." [Opening line, episode 1]
"Good 'ol New Mexico. We're really making a name for this state; first they invented the atomic bomb in Los Alamos, now we've invented mean-ass aliens that teleport out of nowhere! I'm not sure which is worse! Y'know, when they invented the atomic bomb, they were afraid it was going to catch that atmosphere on fire and burn up the whole Earth. But they did it anyway. That took balls. Not us though, the only people taking the risks were the ones who didn't understand them in the first place! We're not brave, we're just stupid."
"Ah, tick-tick-tock...is that the sound of a Geiger counter or my lifespan counting down? It's both! That's right, here at Black Mesa when we talk about Half Life, we mean it in more ways than one. So make your peace and come to Black Mesa! *blasts zombie with shotgun* Here you'll win a chance to fight freaks of nature; escape countless safety hazards; wander aimlessly for hours; and die scared, tired and alone!"
"Okay, I rock. Now stop making me prove it!"
"THIS IS A BAD EXPERIMENT! WE ARE BAD PEOPLE! WHY DID WE USHER FORTH THE GREEN APOCALYPSE?!"
"I know its obvious I'm a genius, but is everyone else really this stupid?"
"Radiation is the gift that keeps on giving."
"Give peace a chance! Or at least stand still!"
"God; no matter how hard I try, I can never ignore gravity."
"I would feel bad about this, but morality is for people who don't have other people trying to kill them every five minutes."
"I'm on your side, you fucking idiots! How many of you do I have to kill before you understand that?!"
"I actually don't hold a grudge against someone talking trash about me if they're shot afterwards. I try to rise above that and find my own peace."
"I mean, there is no actual right to privacy, but it's implied by the rest of the Constitution...and this gun."
"Is that an MP5? It is! Now I can solve up to 800 problems a minute!"
"See, this is why I'm such a good theoretical physicist. I solve problems that shouldn't even exist to begin with."
"I'm probably carrying a few blood-borne diseases on my suit. I'm a walking C.D.C. nightmare. It makes me want to hug someone."
"Well, hey! Now I know all the big secrets! This is why security was so tight at Black Mesa - we were killing people by the hundreds! The best part is there has been no evidence at all - NONE."
"Stop that noise! I hate that! It reminds me of that dream I have where I'm strapped to a gurney watching Fraggle Rock with flashing lights on either side of me, then I realize-I'm in hell!
"See, I believe you, but does my gun believe you?"
"Is this a joke? Nothing's immune to explosives! ... Is this a god? Am I fighting GOD? IS THIS WHAT GOD LOOKS LIKE?!?"
"Well, I can't say I'm happy to return to that guy, but him seeing me again should at least piss him off"
"Yeah, feel the sting of Defense Department funding! I don't know what this gun is, but it's VERY, VERY EXPENSIVE!"
"You're threatening me, you took my weapons, and you're a fucking Time Lord. This is bullshit." [Final canon words, Episode 68]
—All of the above by Gordon Freeman