Moss graffiti - mix yogurt, water and live moss in a blender, then paint onto a wall. Moss will grow in the shape of whatever you painted. Just remember to spritz it with water every once in a while to keep it alive.
Isaac Cordal, recently featured on Wooster Collective, is definitely catching my eye as of late. Please check out his work. This tiny piece is located in a puddle on the streets of Berlin. Such humble art.