In addition to a lot of predictable Banksy spottings, there are some interesting images in this Flickr group, including this one:

Group description:

This group is interested in how people use, abuse and subvert ‘public’ spaces. Now that we lead sedentary indoor lives, public spaces are often neglected or strictly controlled and regulated.
We are interested in how public spaces can be used for ‘unexpected’ purposes other than their design or how that are ‘supposed’ to.