Geocaching is a treasure hunting game where you use a GPS to hide and seek containers with other participants in the activity. Geocaching.com is the listing service for geocaches around the world.
GPS Visualizer (GeoVisualizer) is a free utility that creates customizable maps and profiles from GPS data (tracklogs & waypoints), addresses, or coordinates.
GSAK is the all in one Geocaching and waypoint management tool. Major features include: Multiple databases, sending/receiving waypoints to GPSr, Google maps, conversion to many mapping formats, PDA output (including CacheMate support), HTML...