Geocaching is an outdoor recreational activity, in which participants use a Global Positioning System (GPS) receiver or mobile device and other navigational techniques to hide and seek containers, called " geocaches" or "caches", at specific locations marked by coordinates all over the world
For your phone:
Guide to buying a GPS device:
GPS buying Guide
GSAK (Geocaching Swiss Army Knife)
GSAK is the all in one Geocaching and waypoint management tool.
A site with the goal to provide statistics and tools for all the geocachers in the world.
Simple and Powerful geocaching for iSO .