Position: Front Porch
Okay, I admit this is something more for trick-or-treaters than it is for haunted houses, but I just loved this one when it was pulled on me.
Once my heartrate had dropped, that is.
This works best in a moderately busy neighborhood, where there is a somewhat steady stream of kids, but not so many that the next group can see what happens to the previous one. It's the ultimate in simplicity. On your dimly lit front porch (which was more of a front walkway in my 1950s-era ranch-style neighborhood), there is a chair in which a badly stuffed scarecrow is slumped, straw hat over the face. When the kids walks by, the scarecrow jumps up.
All you need is old clothes, some rope, a hat and some straw, and a person who can hold very, very still.