is a game that's played all over the world, a treasure hunt that anybody can get in on as both a hider and a seeker. There are over 1.5 Million Geocaches hidden worldwide, with over 5 million players.
To play you need some form of way to navigate to provided co-ordinates, a GPS is by far the easiest and most common, but for most urban areas you can try to find them just by looking at the map; smartphone apps are also available for ~$10 if you don't want to shell out for a GPS.
Anybody else here do it? I might hide a cache near Fanime Con 2012, probably bring along a trackable or two.