I found some commands over at the steam forums that turn off the whitish grey haze seen in Left 4 Dead, greatly reducing visibility. It's very hard to see more than 5metres in front.

sv_cheats 1
fog_ovvride 1
fog_colour 0 0 0
fog_max_density 0.95
fog_start 0
fog_end 0
sv_cheats 0

Only works in single player (or servers hosted locally), but makes the experience much more terrifying (though i am easily scared ). Turn off your flashlights for best results.