a night vision scope
Night vision scopes and video cameras are especially useful for ghost research. Not only do they allow you to monitor equipment in the dark, but they also permit you to see some into the infrared realm.
For example, if you push a button on a TV remote, and view it through a night vision scope, you can see a flashing beam of IR energy that bursts from your remote to control the TV. This is a common energy, beneath your nose every day--yet, without the proper technology, you cannot perecive it. How many other things are around us each day that we can't usually perceive?