I'm thoroughly disgusted. I believe I just got done with a game of L4D that had a hacker in it. It started out with a well organized enemy team just beating us rather badly in versus. Then while I was a hunter he turns around and head shots me while in a mid air ponce with dual pistols, which just striked me as lucky (and I'm still not 100% sure if he was or not, it just seems SO unlikely to have someone hack in L4D...) but later, nearing the roof of the hospital we have a smoker set up so he can choke people when they stick their heads out from a relatively safe distance. So, he gets one and messes him up a bit, then a molotov from the bottom of the ladder flies up and lands RIGHT on the small building the smoker was on and sets him on fire. Now either this guy is a god is grenades in L4D or he's using some sort of exploit/hack.
I honestly thought he wasn't hacking at all until I saw that, also a teammate said he was aiming at him through a wall and when he stuck his head out it got blown off, so I suppose he was.