This is by far the most popular item I’ve ever loaned out to friends and family:

Fluke 62 Mini Infrared Thermometer

It’s a digital thermometer which is able to read the surface temperature of an object by simply pointing at it. It’s great for determining the temperature of hard-to-reach surfaces as well as objects that are close. It has a small red-laser for accurate targeting of a surface.  You can use it to check the temperature of your oven for example to verify the accuracy of the oven’s set temperature (they’re often less accurate than you’d think).

I, and many others, have used it during a cold winter to try to find cold air leaks, cold spots, poorly insulated walls, and electrical outlets (and more!).

I’d highly recommend it.