Being an avid looker, I've noticed some awesome shapes made by cracks and stains on walkways. I spent one summer outlining stains in Trinity Bellwoods Park with with pink and blue chalk. Juliana Santacruz Herrera fills her cracks with yarn.
Think all graffiti is bad? Check out Guerrilla Knitting- it's about using one of Britain's retro hobbies to brighten up our world! Knitters use their surroundings as a canvas to showacse their art. Check out Knit The City here: http://knitthecity.com/ Want to have a go yourself? Get some basic knitting skill down here: http://www.ukhandknitting.com/learn_to_knit.php