SmartWater is a fluid you can apply to valuables such as jewelry, electronics, and vehicles. Although invisible to the naked eye, it fluoresces under UV light, and its manufacturers claim that it is nearly impossible to completely remove. Each batch contains a unique signature of "micro-dots" which can be used to trace stolen property back to its owners.
Even though I had never heard of this, apparently it has enough of a reputation among thieves that displaying signs saying goods are tagged with SmartWater was a better theft deterrent than CCTV, burglar alarms, and security guards.
Amusingly, there is also a version of SmartWater intended to be thrown on bank robbers and other criminals to link them to a crime scene.
(via Bruce Schneier, SmartWater Works)