Geocaching

A bit like meetup.com or bookcrossing in that the website provides a means to link physical space and commentary in virtual space. The basic idea is that it's a treasure hunt. You download the GPS coordinates to 'caches' (=treasure) on geocaching.com, set out and find the treasure... It seems to turn into a country walk once you leave the carpark but a country walk with a purpose. It seems to be pretty huge-- there are 210,284 active caches in 217 countries... I read about it in the Guardian.
Like the other two examples, the activity in physical space wouldn't exist without the activity in 'virtual space', it's an integral part of it.