Global Positioning System, or GPS, is a network of Earth-orbiting satellites that transmit on precise frequencies that allow handheld devices like the Garman GPS-V to determine a person's global location accurately to within a few meters. A good introduction to the technology is found here: http://www.oreillynet.com/pub/a/wireless/2000/12/08/gps_intro.html - BillZimmerly
The widespread availability of consumer gps receivers has lead to the formation of several location hobbies...
http://geocaching.com -- GPS users locate 'treasure' caches stashed all over the world. All the cache visitor is asked to do is if they take something they should try to leave something in return. See GeoCaching.
http://www.zefrank.com/sandwich/ -- locate two positions on the globe exactly opposite each other. photograph a piece of bread on the ground at each position.