Bells are an inexpensive method of detecting the presence of a ghost. The use of bells as an interface with the spirit world goes back hundreds of years.
"Bell, book, and candle" is a phrase that has been in use since medieval times to indicate travel to the spirit world, banishing a spirit to Hell, or calling a spirit forth from the darkness.
The medieval church believed that the ringing of bells would frighten away evil spirits. This belief gave rise to the tradition of the "dead bell," a large hand-held bell which was rung at funerals in Britain through the 19th century. Ringing a bell to scare off the evil spirits gave the soul of the deceased a head start on its way to Heaven, lest the evil spirits grab it and drag it down to Hell.
The clear ringing sound of a bell is believed to be purifying or granting good luck in many cultures. For example, the "singing bowl" of Buddhist traditions - an upside-down bell that is rung to grant health or good fortune.
From a modern ghost hunter's perspective, ghosts are said to either jingle bells by accident, or to ring them on purpose out of curiosity. To create a ghost catcher, just hang a lightweight jingle bell by a length of string, ideally in a doorway or at the head or foot of a set of stairs. You may also want to hang them anywhere that a ghost has been reported. For example, sometimes ghosts are said to pass across the middle of a hallway, where a doorway once stood - this would be an excellent place to hang a bell.
With the homeowners permission you can either use a thumbtack to secure the string, or tape the string to the ceiling (ideally with Blue Tape, masking tape, or some other non-damaging tape). If there is a door which often opens or closes on its own, you can hang a bell from the doorknob. This will alert you to any movement of the door, even if you are in a remote part of the house.
If you believe you are dealing with an intelligent spirit, you can ask aloud for it to ring the hanging bell. This is an easy way to communicate with ghosts, in the absence of expensive equipment like an EMF detector.
Bells can also be part of a more elaborate system of signaling. I found instructions for this DIY UFO detector which could also be quite useful for ghost hunting.
One caution about the use of bells to hunt ghosts: they can be very susceptible to breezes, and even drafts. Be sure that all exterior windows and doors are tightly shut before you begin your investigation. Even so, you may want to use a small flame (like a disposable lighter) to ensure that there isn't a draft before you hang the bell.
Photo credit: Flickr/~K~