There are a lot of technologies out there capable of producing fog, mist, and haze.
Water-based hazers
are a popular technology used for mist, haze, and thin fog effects.
Its primary use is for lighting effect enhancement:
a backline of PARs on a big stage simply puts a bunch of colored spots on the floor,
but add haze and the light beams become visible, making a huge wall of colored light.
Haze is great for adding a spooky atmosphere to a haunted house as well.
This page is concerned with buying water-based hazers...