You can use a thermal imaging camera for a lot of non-obvious tasks e.g.
- Tracing the path of water from a leaking roof. The water will show a temperature difference which will let you trace from the point of damage (usually a brown spot on a ceiling somewhere) and the source of water ingress.
- You can find the locations of studs and joists inside your walls. This works particularly well on an external wall with crappy insulation (I live in an old, leaky house). You can also try turning up the heat/AC on one side of the wall and waiting a little bit.
- You can trace the forced-air ductwork in your house. Turn on the heat or AC and let it run a while. The temperature difference of the cooled/heated joist bays will stand out.
They're a huge help in debugging electronics.
- If you're building low-power electronics, you can put your device into its various power modes and see which ICs are still burning power. This technique helps identify if you forgot to turn something off.
- You can identify a shorted decoupling capacitor on a board because its the one generating heat. This is very helpful when fixing old equipment as the capacitors are very likely to have died.
Also, cats under IR: they are interesting (and evil).