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)