UPDATE: This paper is no longer online. I only posted it online for a period of time.

• EMF readers: These items are used to record and document disruptions in the electromagnetic field of a location. These devices don't detect ghosts but record high levels of electromagnetic particles. High levels can be found near strong power sources like power lines or old or faulty wiring. The reason ghost investigators use these readers is because it is known that ghosts/spirits feed on any kind of energy source they can get. Ghosts/Astral beings need energy to disrupt the location, either by moving objects or actually appearing as some sort of form like a random light appearing (also called and orb) or a shadowy figure. Batteries are a good source for them to feed from. And when there is no logical or natural source of a high level EMF reading then most likely the EMF is picking up on a phenomenal occurrence.
• Voice Recorders: Voice Recorders are used by ghost hunters and investigators to pick up disembodied sounds or voices that appear on the recorder or tape in which cant be explained by the investigating team. Normally they use these devices to communicate with the spirits. The reason these devices can pick up on these sounds is because spirits need energy and then they use that energy to communicate. Think of it as radio waves or energy waves. For example, radio stations send their signal out through air waves, but we cant hear them unless we have a device that can pick them up so we can hear, thus called the Radio. Voice Recorders in this manner work similar which is called Electronic Voice Phenomena (EVP)
• Digital and 35mm still cameras: These types of cameras are good for capturing quick snap shots of a location. Sometimes pictures can contain or display images that would not necessarily be in the photo. For instance, a photo can contain what would look like mist, fog, smoke, a figure or some type of light anomaly that was not there to begin with. 
• Video Cameras: These devices are very useful for capturing any kind of strange phenomena. 
• Digital Infrared cameras: These devices are able to capture images invisible to the human eye. These images can be loaded to a computer or another recording device
• Thermal-Imaging Cameras: These devices are used to capture heat sources that are invisible to the human eye, even in darkness. Heat sources are useful to capture presents of ghosts. We're back to that energy thing again. Heat comes from energy and since ghosts need energy, heat will also follow.
• Dowsing Rods: These devices have been around for thousands of years and the mystery behind them is still unknown, but they are proven to be uncannily accurate. How they work is you hold one rod in each hand by the handle and point them forward. When walking around the rods should point in the direction of any energy source and when they cross you are in that particular energy source.
• DVRs and laptops to record the data: These devices are useful to capture, document and store any data from the investigation.