Hand- (and foot-) prints are one of my favourite examples of what thermal imaging can reveal. In the second photo, you can see that the effect is different when the surface has been pre-heated (in this case, by a laser printer).

Chilled objects can be especially interesting. Notice how the bottle of Coffeemate® and the carton of half-and-half aren't thermally transparent (you can't see what's behind them), but it is still possible to see the fluid level in each because it is cooling the container from inside. The thermal image of the shopping bag reveals where the glue used to attach the carrying handle is, in-between two layers of paper.

Thermal imaging of clouds at night creates an otherworldly effect. In the third image, a semi-infamous homeless camp in a wooded park is captured — the residents were completely invisible to the naked eye, but show up like lightbulbs in thermal wavelengths. That specific example made me realize why the military and police find these devices so useful.
The fourth image is of the Museum of Glass in Tacoma, Washington. Its central cooling tower apparently works very well.

Just about any electronic device gives off a significant heat signature. Some LCD monitors' transformers and lighting systems give off so much heat that their shape can be seen through the case. Similarly, in the last photo, a Cisco switch whose cooling fan has been removed runs so hot that the locations of its major chips an also be seen from outside the casing.

A room full of hidden cameras (part of the traveling Spy: The Hidden World of Espionage exhibit) has its difficulty level dramatically reduced with a thermal imager.