Over the last few years the price of fog machines has plunged to the point where you can pick up a decent machine for just a couple of bills. The cost of heavy-duty, high-end models is still in the triple digits, but for seasonal use an off the shelf unit from your local mega-mart is more than adequate. They produce a fine mist that adds a lot of atmosphere and enhances lighting effects, but they're not very effective at producing the ground hugging fog that helps define the look of classic horror films. Normally that kind of fog requires the use of dry ice, but with a little tinkering you can get a similar effect using your existing fog machine and a fog chiller. T3Hprogrammer at Instructables just posted a great how-to article that shows how to build a basic fog chiller for around $10.
Fog machines are great for Halloween... but fog tends to rise up into the air, spread out, and disappear. If you do not have a very powerful fog machine, this can ruin the whole effect.
The solution to this is to build a fog chiller. The fog chiller cools the hot fog from the machine, causing it to stay low and billow along the ground.
This is a small fog chiller, which works well and is cheap. It takes an hour or two to build.
Get hoppin' and you can have it ready by Halloween.