GeoCaching is a worldwide treasure hunt game for GPS users. The basic idea is that people hide caches all over the world and share the locations of those caches on the geocaching website. Anyone with a GPS device can then go find these caches and, if they're lucky, find a bit of treasure. You can find out everything about geocaching at: http://www.geocaching.com.
Geocaching is fun to do alone, with a friend, with kids, with your dog, while you're traveling, or as a club. You can look for caches or you can hide caches of your own. You can even sit around and be part of a forum about geocaching. But really, it's more fun to go out and look for caches!
Here are some more GeoCaching resources: