"Oh crap, I'm late." [Opening line, episode 1]
"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!"
"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."
—All of the above by Gordon Freeman