Everyone's favorite Gorgon...

...that is if you  like being turned to stone.  In all seriousness the image of Medusa's head  was used in antiquity as a symbol of protection, this stems from a legend involving Perseus, who cut off Medusa's head and used it as a weapon against the Hydra (who I drew awhile ago).  When he was done with the head he gave it to Athena (I'm assuming he did this because it didn't match his drapes) and she put it on her shield as protection.