For the record, I don’t endorse vandalism, but some of these are really rather cool:
From Flickr user yorkshire4 | From Flickr user gfrancis
From Flickr user mmmazzoni
From Flickr user darnolds
From Flickr user loveandunrest
From Flickr user rojotelefono | From Flickr user cherrybiz
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ame72 is the original lego street artist, there are some great works on the http://www.ame72.com website or have a look at ame72 on flickr.