Sharpie

Infra-red analysis of the Mona Lisa in the Louvre uncovered the fact that "THIS IS FAKE" was written in Sharpie underneath the paint. (TV: City of Death ^[+]Loading...["City of Death (TV story)"])

Cleo Proctor wrote the Doctor's name on her arm with a Sharpie to keep her from forgetting it. (AUDIO: Lost ^[+]Loading...["Lost (audio story)"])

Behind the scenes

An illustration of a Sharpie.

A piece of Redacted fanart officially acknowledged by the Doctor Who Twitter account featured an illustration of a Sharpie.