Currently avalible for GPS enabled Pocket PC devices and the Garmin Colorado and Oregon models.
From Groundspeak, Inc. The makers of Geocaching.com and Waymarking.com
"Wherigo is a platform that allows you to build location based GPS experiences on your computer and play them in the real world. Imagine playing Zork, Secret of Monkey Island or Myst, but in the park around the corner, or on the beach during your family vacation. Rather than clicking the mouse and selecting a location to move your character, you physically move from one location to the next to advance the story. Rather than searching for puzzle clues on a screen, you look for them in the real world. Using Wherigo, you can create interactive tours, adventure games and puzzles... the possibilities are endless."
"Markeroni: A friendly, informal and light-hearted online community where history fans, treasure hunters and travelers log their visits to historical markers and historic landmarks. "
"Waymarking is a way to mark unique locations on the planet and give them a voice. While GPS technology allows us to pinpoint any location on the planet, mark the location, and share it with others, Waymarking is the toolset for categorizing and adding unique information for that location. Groundspeak's slogan is "The Language of Location" and our goal is to give people the tools to help others share and discover unique and interesting locations on the planet. We invite you to share your part of the world with us through Waymarking.com."
"This website is an illustrated searchable online catalog of historical information viewed through the filter of roadside and other permanent outdoor markers, monuments, and plaques. It contains photographs, inscription transcriptions, marker locations, maps, additional information and commentary, and links to more information. Anyone can add new markers to the database and update existing marker pages with new photographs, links, information and commentary."