Geocaching.com

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

geocaching.com

For your phone:

Mobile

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.

GSAK

 

Project-GC

A site with the goal to provide statistics and tools for all the geocachers in the world.

project-gc

 

 Cachly
Simple and Powerful geocaching for iSO .
Cachly