This is a great blog. Full of genuinely funny examples of street art, improv, pisstake mashups... it's amazing when you look around at the urban landscape and see the way people are compelled to leave their mark, make a statement, edit their environment.
http://urbanprankster.com/
Grains of Truth
Guardian story on rice mound exhibition
http://www.guardian.co.uk/stage/2008/sep/17/theatre