The Lenstag App allows you to register your own gear and check the serial numbers of cameras/lenses you are considering purchasing to see if the items in question have been flagged as stolen in the Lenstag system.
And if the worst happens to you, the Lenstag App allows you to quickly flag your gear as stolen from your mobile device.
For what it's worth, all my cameras and lenses are registered with Lenstag. I highly recommend taking advantage of this free, valuable service. Check out the Lenstag website for more information. [Sean]