Thieves are always on the lookout for camera equipment. Like jewellery, cameras and lenses are easy to resell, portable, and net a high price. Lenstag is a free service that aims to combat photography gear theft.
The service is essentially a camera gear registry. After signing up, enter the cameras and lenses you own, along with their serial numbers. You'll then need to verify you actually have this equipment by uploading a photo of the item with its serial number in clear view.
In the unfortunate case your camera is stolen, you can use Lensgear to report the theft, and the site will post the stolen item's serial number and model so it shows up in internet searches. Prospective buyers of possibly stolen camera equipment can be warned by looking up the serial number on Lenstag or just searching online.
Obviously, this will only work if people are concerned about possibly buying stolen equipment and bother to do the quick search, but it's a small step to solving this huge problem.