A letter opener is a sharp metal object that is used to open envelopes and is occasionally wielded by hunters as a weapon.
While hunting a shapeshifter after having lost his weapons, hunter Dean Winchester located a silver letter opener. As it was silver and sharp, Dean appropriated the letter opener to use as a weapon against the shapeshifter. Dean eventually found a second silver letter opener that he armed Sam with. During a fight with the shapeshifter, Dean uses the letter opener as a weapon in combat and ultimately kills the shapeshifter by stabbing it in the heart with the letter opener. He leaves the makeshift weapon there where it is found by a SWAT team that discovers that shapeshifter's corpse.
After being called on a case in a mental hospital, Dean steals three silver-plated letter openers from three different nurse's stations to kill the Wraith that they are hunting. During the hunt, Sam slashes Doctor Aaron Fuller's arm after suspecting that he is the Wraith that they are hunting. However, Martin stops him and points out that Doctor Fuller's arm is not burning from contact with the silver. Martin later slashes the Wraith with his letter opener and Dean ultimately kills her by stabbing her in the heart with his.