Graffiti, as the name suggests, will let your friends draw freehand on your Facebook wall. If you want to paint graffiti on the wall of your Facebook friends, they'll have to add the Graffiti application to their Facebook profile.
Graffiti has a Flash based drawing canvas where people can scribble using paint brushes of various widths and colors. The drawing is rendered as an image and added to the Facebook wall. Neat.
Graffiti Wall on Facebook - Add to your Facebook profile.
Related: Draw Graffiti on Street Walls with Spray Paint and Brushes