See something or say something - Eric Fischer's flickr/Twitter maps
I was going to write up some thoughts on what we can learn about cities from Eric Fischer's beautiful new maps of tweets and flickr photos (London above), but this interview with Eric pretty much covers it. I would just add that 'flickr density' might be a useful proxy for amenity value in economic or geographic analysis.
You should also go enjoy the full set of maps here. The man's a genius.