From the Geohashing homepage: "Geohashing is an adventure, comprising journeys to random places within a given area. Every day, effectively random locations are generated by an algorithm that achieves randomness using stock market data. (...)"
Geohashing can easily be combined with mapping.
More information and tools:
- Info: http://geohashing.org
- Coordinate calculator: http://geohashing.info
- Small Hash Inquiry Tool: http://bit.ly/smanthill
- Geohashing Poster Tool: http://bit.ly/ghposter
- OpenStreetMap Achievement for Geohashers: http://wiki.xkcd.com/geohashing/OpenStreetMap_achievement