OpenStreetMap.org is like wikipedia but for maps, you can add anything you like and help make a free map of the world.
Mapping parties are community events targetted at getting a specific area mapped in a day or a weekend. You come along, are loaned a GPS unit and shown how to use it (it's easy!) and you go out mapping an area of your choosing. Then you come back and are shown how to get that data in to the map.
They are relaxed events and you're supposed to have fun. Often we grab a beer after the event, and this is no exception!
The venue is near CalTrain, check the wiki page for more info.
Official Website: http://wiki.openstreetmap.org/index.php/San_Francisco
Added by SteveCoast on September 8, 2008