I am currently teaching a year 11 visual art class focusing on digital imaging: activist art so am collecting relevant websites. From seedbombing to guerilla girls, adbusters to the f64 club, learning to love you more eco-activists the worsted witch and the gang who brought you park(ing). Here's a great how to be a guerilla artist guide from writer and illustrator keri smith.
"Guerilla art is a fun and insidious way of sharing your vision with the world. It is a method of art making which entails leaving anonymous art pieces in public places. It can be done for a variety of reasons, to make a statement, to share your ideas, to send out good karma, or just for fun. My current fascination with it stems from a belief in the importance of making art without attachment to the outcome. To do something that has nothing to do with making money, or listening to the ego. "