A way of social metadata and data integration: tagging by humans.
If you have a photo of somebody holding a camera, that somebody may also have uploaded that photo, which can be tagged to your photo. Now If you again take a photo of someone else…
Here a nice example how a chain can be started. Note the great idea of tagging the related photos by notes on the camera:
http://flickr.com/photos/anjeve/262175558/
(all rights reserved on that one, cannot blog it directly, click link)