A curse, or hex, is a type of spell work done to cause misfortune, harm, or death to whomever it is placed on. Sometimes a curse can be broken by killing the caster, other times it can only be broken through some form of reversal spell or ritual.
Euchee Tribe Curse
Two hundred years ago in Oasis Plains, Oklahoma, the American cavalry came to relocate the Euchee Tribe. The Euchees were resistant to being taken from their land. This caused the Calvary to become impatient and on the night "the moon and the sun share the sky as equals," the cavalry raided the Euchee village, raping and murdering the tribe. These rapes and murders would go on for six nights until every man, woman, and child still in the village was dead. As an a final act of retribution, on the sixth night, the dying village chief whispered to the heavens, "that no white man would ever tarnish this land again." The valley would be protected by nature, which would bring as many days of misery and death to the white man as the Calvary had brought upon the Euchee people. The Winchesters believed the curse to be unbreakable and the only option to be to avoid it.
A hundred years ago, in Baton Rouge, a conjure woman created the rabbit's foot under a full moon, in a cemetery, on Friday the 13th. The cursed rabbit's foot would grant the possessor exceedingly good luck, but once the foot was out of the hands of the owner, the luck would turn, eventually leading to death. The curse was eventually stopped by Sam and Dean by destroying the rabbit's foot in a cemetery using a "heavy-duty cleansing ritual" that includes bone ash and cayenne pepper.
The Tiamat Talisman was a coin created in ancient times by priests of the god of primordial chaos, Tiamat. The coin has the power to grant any wish a person may make in a wishing well, however eventually the wish will become warped and "backfire." To break the curse and remove all the wishes made, the person who placed the coin in the wishing well must be the one to remove it.
After a young woman named Corey performed a ritual that summoned Veritas, the goddess of truth, all people in Calumet City, Illinois, who asked aloud for the truth invoked her curse. Once invoked, the person is bombarded with harsh truths until they are driven to suicide as a tribute to Veritas. The curse is broken with Veritas' death.
As punishment for stealing fire from Mount Olympus and giving it to humanity, the Greek Titan Prometheus was chained to the top of a mountain and cursed by Zeus to die and have his liver eaten by an eagle every day for eternity. In 2006 an avalanche freed a now-amnesiac Prometheus, who met a young woman named Hayley and produced a demigod child named Oliver, who was also afflicted with Prometheus' curse.
The curse was finally broken in 2013 when a sympathetic Artemis shot one her arrows into Prometheus, allowing him to drive it into Zeus, killing both and freeing Oliver from the cycle of death and resurrection.
The Mark of Cain was both lock and key, created by God to keep his sister the Darkness imprisoned. The Mark was entrusted to Lucifer, but it soon began to assert its own corrupting will. Revealing itself as the first curse, it enhanced the bitterness and envy that Lucifer held for humanity and pushed him to act on it. Lucifer passed the Mark onto Cain after forcing him to kill his brother Abel, and Cain in turn passed the Mark onto Dean Winchester. The Mark slowly begins to corrupt, amplifying the darkest compulsions of the bearer. If a human bears the Mark, they must continually feed it, otherwise they will succumb to the side effects of its immense power. If the bearer were to die, they would be resurrected as a demon. To remove the Mark without having passed it along to another bearer unlocks the cage that holds the Darkness, releasing it upon the world.
Kiss of Death
The "Kiss of Death" is an Aramaic curse created by the witch Sonja. Once the ritual is performed, it must be sealed with a kiss. The kiss marks the person for a qareen, who will appear as the person's deepest, darkest desire before ripping their heart out. The curse can be transferred by kiss, but once the qareen has killed the cursed individual, it will work back until it gets to the original person who performed the curse. The only way to lift the curse is by finding the qareen's heart and stabbing it.
The "dearmad" (forget) spell is a Celtic curse used by Gideon Loughlin on Dean Winchester. When a person is hexed by this spell, they will slowly begin to forget things. As the curse progresses, memories of people and places as well as themselves will begin to fade. Eventually the afflicted will forget how to speak and swallow, resulting in their death. The curse can only be lifted by performing a ritual found in the Black Grimoire. After the deaths of the Loughlin Family, Rowena was able to perform the reversal spell upon Dean and save his life.
A curse mentioned by Rowena that could cause the afflicted to lose their memory as Dean did, one of the known symptoms of this curse is the afflicted will start to lose all their body hair from the neck on down. Rowena stated to break the curse, all that had to be done was to kill the caster.
While facing a Kohonta, the Winchesters learn from Sheriff Mason Romero that a Kohonta is a human cursed to be transformed into a monster with an insatiable hunger for engaging in extreme cannibalism. The curse was performed by Native American shaman, in this case on a young white settler named Henry Parker who ate his own family and some of the local tribe before he was cursed. There is no known way to break the curse other than to kill the Kohonta with a silver blade through the heart.
At some point before her death, Rowena cast a powerful curse upon her flat that would kill anyone that entered, causing them to bleed out within a short period of time. This curse killed the witch Jacinda, but didn't harm Sam, whom Rowena had specifically excluded from the effects of the curse when she cast it. Sam was forced by the Witch Mother and Emily to get what they needed for a resurrection spell from the flat as they could not enter to get it themselves or break the curse. Dean was later able to enter unharmed as well to burn Jacinda's body, having presumably also been excluded from the curse's effects by Rowena.