Quite often. And this isn't me bragging, it's me complaining because it gets damned annoying. Especially when it leads to me being kicked or banned from a server.
Whether it be in Left 4 Dead, Counter-Strike: Source, Bad Company 2, Quake 3, Starcraft 2, Halo, or any other online game I play decently at, it gets really old really fast being accused of hacking or cheating. Especially when I see absolutely blatant hackers roam free with nary a finger pointed towards them. (and these blatant hackers are usually really bad, even with the hacks, cheats, or exploits)
And, I'm not even that good. I've played in pro-level scrims and tourneys. I often fair well in them too. But I'm no where near as good at those games as the people that devote a scary amount of time and practice perfecting their play. Maybe that's why I get accused of hacking. I play better than the average player, but I don't have a thousand plus hours logged in the game(s).
I suppose it could also be that 90% of online gamers are juvenile, whiney pricks with VERY fragile egos.
Yeah, I'm gonna go with the latter.