A good thermometer can detect changes in room temperature quickly. It's best to use the old fashion mercury filled, red line, thermometers. They have electronic thermometers but in ghost cases where electromagnetic forces have been reported they might fail right when you need them.
Great for navigation if you know how to use one and also great for picking up those electromagnetic forces. A compass will react to any magnetic or electrical stimulus that is out of the ordinary. Same rules apply here, try not to use the electronic compass's. Stick to the simple boy scout, needle points North version.
(also called, a Spirit Wind Chime)
The idea here is that the spirit will pass by and cause the ghost catcher to chime much like a wind chime does in the wind. You can use the wind chime that you get in the stores, however, try to get the ones made of the lightest material. You don't want the arms to be too heavy. For obvious reasons these will not be affective outside or in any area where a breeze or wind is blowing.
You can make Ghost Catchers yourself. Take about 8 to 10 very thin strips of metal (about one inch wide and about 6 to 8 inches long). Put a hole in one end of each strip. Tie seperate strings to each strip. Leave about 6 inches of slack in the string and tie the other end of the strings to a pole or something that keeps the strips hanging down. Tie them about one half inch apart on the pole. The strips shouldn't touch each other but shouldn't be very far apart either. Hang the pole from the ceiling or in a doorway. I know that some people paint the strips with glowing paint to make them easier to see at night or in the dark. Any variation on a wind chime will work. For the best results make several ghost catchers and put them at different spots throughout a house.
Infrared Thermal Scanner
These saves on infrared film and time by accurately pin pointing the cold spots. These scanners are excellent to use in outside investigations. Price range is around $300 to $500.
Air Ion Counter
Measures positive and negative Ions in the air. This little add-on is a great instrument to have for use in the field. The price is high, around $400 - $600.
When you have a team of three or more people headset commincators are a great idea for staying in contact when spread out. You can use the hand held walkie talkies but the headset frees up your hands for holding cameras etc. They will run in the price range of $30 to $80 a piece. I've seen good sets that work up to a distance of 100 yards for 40 bucks.
Small battery powered spot lights really help at night when it comes to setting up and taking down cameras and other equipment. They can also be used for safety and to get a better view of the surrounding terrain at night. Get the lights that sit on the ground but have swivels on the lights to set them to different angles. Plan on getting between 2 and 6 spot lights depending on your needs