Geocache is a game created for users of GPS. "Treasures" are hidden and the coordinates are made available for people to find. You can create an account and keep track of your found treasure on the website.
Geocaching - The Official Global GPS Cache Hunt Site
Waymarking.com provides tools for you to catalog, mark and visit interesting and useful locations around the world.
GeoWise More information on geocaching
National Geodetic Survey Up to date information on benchmarks
Earthcache
Created by the USGS, Earthcache is similar to geocache. Instead of searching for stored containers, you are discovering Earth's natural treasures.