At the risk of labouring the point it is not just the BIG street art that is worth seeing. There are hundreds of hidden gems, little pieces that you could just walk by. The work of Gregos for example. Painted casts of Gregos, in different moods, can be seen all over if you look hard enough. Here's a couple from Shoreditch.
baby face, overpainted by The Rolling People
Here's another way that the street furniture of the modern metropolis is being subverted. Blue Plaques remembering actual local people who had enough friends and influence to earn their own unofficial blue plaques.
Urban Still Life, Yipi Yipi Yeah
These two are from Madrid's Yipi Yipi Yeah. Some of YYY's other work is more political and interesting. These look as though they should mean something, be a telling comment about urban life or factory farming or some such. Or maybe they're just an exercise in colour and text.
This next one IS a comment on the cruelty of factory farming.
Paul Dan Smith
Here's some cat art from the streets
And here's some portraiture
Ben Wilson, Chewing gum graffiti
this is painted on chewing gum, stuck to the pavement. Painted in situ. With a ridiculously thin brush. What can I say?
Done and done