The idea behind this app is that instead of tagging actual buildings, you're tagging virtual buildings. Whether this will reduce crime (or actually encourage it) is undecided, but the app has a neat augmented reality Foursquare tie-in.
The free app is only available in the US, where graffiti artists can daub their insignia on the sides of restaurants or other retail outlets, and then share them on Foursquare or even Facebook and Twitter. Users can fire up the augmented reality app to check for tags wherever they go, and even steal other people's tags (which awards the creator extra points).
While the app is free, there are several in-app payment options available if you're after an upgraded paint editor or wish to store an unlimited number of tags. Graffing "the curry sucks" on a restaurant is perhaps a more powerful message than a negative message on Foursquare could ever be. [iTunes via Recombu]