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 iOS.
Cachly

 

Michigan Regional Geocaching Wiki

Notes and tips from your Michigan reviewers.

Wiki