Coins can be imbue with magical power and used for a variety of purposes
This was created by Tiamat's priests in ancient times. Using incredible dark magics, they forged the coin to sow the seeds of Chaos. The coin features the Babylonian God of Primordial Chaos, Tiamat. On the other side is the symbol of the Babylonian God Shamash, bringer of light and justice.
A person who tosses the coin into a well effectively "turns on the well." It grants any wish made on the well. Once the wish is granted it begins to warp until the wisher no longer wants it. The coin must be removed from the well by the person who tossed it in in the first place. No one else can remove it. Once it is removed, however, all wishes will be undone.
Sam and Dean discover that wishes made at the wishing well in Lucky Chin's restaurant in Concrete, Washington come true. They drain the wising well and find a very unusual and ancient coin at the bottom. Try as they may, they cannot remove it. They eventually discover that Wes Mondale inherited the coin from his grandfather, who found it in North Africa, but warned him not to use it.
Crowley reveals he knows about Sam and Dean's plan to use the Horsemen's Rings to trap Lucifer in the Cage because of a magic coin his valet planted in Sam and Dean's car when they broke into his house.[5.10 Abandon All Hope] The coin allows him to track their position despite the fact they are carrying hex bags to hide them from demons. It also allows him to eavesdrop on their conversations.
Later in the episode, after Crowley comes back from his rendezvous with the demons, he discovers that they have planted a coin on him as well. They use the coin's signal to send a hellhound after Crowley, Sam, Dean and Brady.
Romanian Hex Coin
A Romanian Cyrillic coin from the 15th to the 19th century which can help focus the power of a spell.
Maggie Stark is angry at her husband Don Stark for having an affair and is killing people associated with Don by placing a magical coin near them and working a spell. Sam manages to stop one attack by shooting the a coin, destroying it. Don strikes back, ruining Maggie's charity art auction and killing her best friend. Finally, Maggie and Don reconcile after some relationship advice from Sam and Dean. However Maggie still tries to kill them, thwarted only by Don retrieving the hex coins from their motel room.