I used a func_dustcloud on a nodraw just make a cube in any room and then tweak the 'size'=100-200 and number of 'particles per sec'=500
though on some machines you could get lag so try to keep the number of particles down and the size too..
edit:also you cna do the same with ground fog too just lower the brush to about 2 feet in height
PS. if you like to make your fog look as real as possible use the fog_control with the Func_dustcloud, or in a dark area


