Serge Seidlitz drawing live for Doodle Weekend at Birmingham Children's Hospital. Members of the public were later invited to colour in his drawings, with the weekend raising money for a new theatre in the hospital.
Japanese artist Sagaki Keita creates composite pen and ink illustrations using thousands of densely drawn doodles to form a bigger picture. The amount of time to draw these must be substantial. You can see more on his website. Via Colossal.