"Friendly fire always has the right of way dude!"
"Hey, you ever get the feeling you're being followed?" (love saying that in Halo just before beating someone round the back of the head hehe)
"I want to say this was too easy, but fate will beat me down if I do"
"You know if they're shooting at you, probably means they don't want to be friends."
"Anything too simple, is a trap.."
"Do you know what the chain of command is? It's the chain I go get and beat you with until you understand who's in command!"
"Ask a stupid question, get a stupid answer" (personal favourite)
"No **** Sherlock!"
"That's so wrong.. on so many levels"
"You've got about three steps, before I beat you to death with whatever is handy" (i tend to say that to my bro alot when he's screaming at his console cause he reckons it's cheating; but also tends to come up whenever i get into an argument in a bar ^__^)
"Man, you have the memory of a newt"
"Did you know, smart-arses have a high mortality rate... just a friendly warning"
".. yeah, i know .. the couch, wish I got frequent flyer miles from sleeping on it; I'd be able to go to Hawaii like I want"
About the only ones that come to mind, but I tend to come out with quite a few more; usually spur of the moment things that I've picked up from somewhere.