ID microchips, no bigger than a grain of rice, can be implanted by veterinarians into dogs, cats and even many reptiles. The procedure is relatively non-invasive and takes moments to complete - a lot like getting a shot. The device holds a number that can be scanned; this number correlates to your ownership information in a national database.
For a limited time, Washoe County Regional Animal Services is offering FREE microchipping for your pet. Although the procedure is relatively low-cost to begin with, some of us don't have the extra budget to make it happen. You can find dates and locations of this free service here: http://www.washoecounty.us/animal .