Photosynth was demonstrated at the March 2007 TED Conference. A video of the demonstration blows my mind -- particularly the segment that begins about 4 minutes 50 seconds into this 8-minute presentation (well worth viewing in its entirety):

Imagine being able to take photographs of any place, building, or object from all the world's digital photos -- and map them together using tags supplied by people who have already viewed the image. The Cathedral of Notre Dame example in the demonstration is a great start... But think of the possibilities, not just globally, but also locally, in terms of bringing your community's experiences and knowledge to bear on any particular local topic.