Urban Prankster points out the unintentional, subconscious art that results from the removal of graffiti: 
I snapped the above photo in downtown Manhattan a month or so ago. There were several stone rectangles around the property that all looked similar. What a great example of the subconscious art of graffiti removal!
Personally, I'm always somewhat disappointed when people paint over graffiti (unless the graffiti is offensive, of course -- although that, too, is in the eye of the beholder). This way of looking at it makes me feel slightly better about it, I suppose. It's all about perspective.
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Very nice site!