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: )

Cleo Proctor wrote the Doctor's name on her hand with a Sharpie to keep her from forgetting it. (AUDIO: )

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