Grime Writer is a detergent-filled graffiti marker that cleans away street-filth to leave your message behind. There's a good chance that the graffiti you create with these is no more legal than any other kind -- there've been successful prosecutions against companies in the UK that paid "street teams" to "reverse-graffiti" their messages by using detergent and stencils to selectively clean away grime from public walls, leaving behind commercial messages.
Grime Writer is a special chunky marker pen that can be filled with cleaning solution, and used to create art on a canvas of filth. Use it to tag your dirty vehicles & windows, or to transform dirt into artistic expression.
Much more socially acceptable than real graffiti and (more importantly) a lot less illegal - Grime Writer helps you leave your mark wherever you find muck. Use responsibly to help promote the phenomenon of negative graffiti, and brilliantly combine the crime of defacing a bridge with the community service time cleaning it up again afterwards, into one harmless, helpful, creative act.
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.