Sigil of Azazel

John uses the Sigil of Azazel in a summoning ritual in 2.01 In My Time of Dying.

Sigil of Azazel The sigil of Azazel is the symbol that John draws as he is preparing to summon the demon. The name Azazel is most commonly used as a reference to Satan or a fallen angel. By drawing the sigil of Azazel, John is drawing a sigil, a symbol that is used for magical purposes, of the devil. – Pad of Definitions (2.01 In My Time of Dying), Official Website

This symbol is also known as the Seal of Saturn. In 3.04 Sin City it is revealed that the name of the Yellow-Eyed Demon is Azazel. John's use of the Sigil of Azazel to call the Yellow-Eyed Demon in 2.01 In My Time of Dying suggests that John knew the demon's name all along.

Sam uses a similar ritual to summon Ruby in 3.16 No Rest for the Wicked.

When Bobby summons Crowley in 6.04 Weekend at Bobby's, the sigil can be seen on his desk where he performs the demon summoning.