In his famous book, Thirteen Steps to Mentalism, Tony Corinda devotes his first "step" to the Swami or Nail Writer; a hidden device that allows you to secretly write predictions. Swami writers come in many styles, most notably:
Swami Writer: a tiny piece of lead cipped under, or onto the nail.
Band Writer: a pliable metal clip that fits on the ball of the thumb, with a piece of lead for writing.
Boon/Bug Writer: a circular disc that has the lead mounted in the middle. The disc sticks to the pad on your thumb.
Thumbtip Writer: a piece of lead protruding from a thumbtip.
Boon / Bug Writers
Popularized by Eric Mason and Barrie Richardson, the boon writer is a small pad that sticks to your thumb to allow you to secretly write your prediction.
Everything Else Nail Writer