The user can give a person a power, an object of power or change nearly any event within their power to something as trivial as transporting them to a location or as simple as knowledge on something. Some other uses include being their servant till they die or something as extreme as giving them a plot coupon.
They can also create their own contract granting something within a contract's payment being as no payment.
The user can take anything from a contracted client be it their powers, emotions, memory, soul or event in their past or future and maybe even someone connected to the client who they can choose to make him or her their servant. The contract can be fulfilled by doing some task or arduous payment.
The user can bend the agreement to their own desires limited only by his/her power and therefore they can change the contracted into a monster as a way to give him or her a power. As long the agreement didn't say they could not perform a certain action their restraints are somewhat relative. The user can also simply agree to grant the person what they want without telling them what their payment is or when they will pay it.
Giriko Kutsuzawa (Bleach); via Fullbring, "Time Tells no Lies"
Nulgath (Adventure Quest Worlds)
Sakura Kinomoto (CardCaptor Sakura)
The Djinn (Wishmaster series)
The Devil/Roark (Ghost Rider)
Rumplestiltskin (Once Upon a Time)
Demons (Ángel o Demonio)
Dyshana (Record of Agarest War)
Mimel (Record of Agarest War)
Malebolgia (Image Comics)
Calvin Dion Avericci (Archie's Weird Mysteries)
Bill Cypher (Gravity Falls)
Ursula (The Little Mermaid)
Onmyoudou (Tokyo Ravens)
Beast Knights (The Lucifer and the Biscuit Hammer)
Black Cat of Contract (Kyōkai no Rinne)
Rokumon (Kyōkai no Rinne)
Mr. Smith (Twisted Tales)
Chains (Pandora Hearts)
God of Death (Yu-Gi-Oh! GX)
Hades (Disney's Hercules)
Dr. Facilier (Princess and the Frog)
Merlin (Nanatsu no Taizai)
Spirits (Re:Zero Kara Hajimeru Isekai Seikatsu)
Artifical Spirits (Re:Zero Kara Hajimeru Isekai Seikatsu)
Magical Girls (Oto x Maho)
Umbra Witches (Bayonetta)
Lumen Sages (Bayonetta)
Lorenzo (American Dad!) made a deal with Roger Smith to teach him guitar lessons that will ensure his victory against Jamal at the cost of his soul...
...but Roger cunningly conned Jamal into covering the "price for [his guitar] lessons", saving Roger from Lorenzo's grasp at the cost of Jamal's soul.
Contractors (Darker than Black) are granted supernatural abilities that they must pay a contract after use, such as Bai's molecular manipulation ability requires her to sleep after using.
Go Okada (Alive: The Final Evolution) can make the Contract with the Shinigami on normal people in that if they break a promise with him, the Shinigami would kill them.
Giriko Kutzusawa (Bleach) breaks a contract with the God of Time set by his Fullbring, Time Tells No Lies, causing his eye to be burned away by the Flames of Time.
Calvin Dion Avericci (Archie's Weird Mysteries) gave unsuspecting teens charge cards that allows them to buy things within his mall, free of charge, until they reach a limit, to which he then turns the victim into a mannequin; he corrupts teens so in order to repay his own debt of being able to shop endlessly, and his failure to do so resulted in his death.
Rei Ogami (Code:Breaker) had to make a contract with the Seven Demons of the Flames to regain his lost mastery over them in order to use their powers again, sacrificing his sense of taste, Yuuki's Sound ability, his heart upon death, his enemy's envy, his sense of smell, and eventually his memories.
Kyosuke Higuchi (Death Note) made a deal with the Shinigami Rem for the Shinigami Eyes, granting him the ability to see names and lifespans of the people he looks at in exchange for half of his remaining lifespan.
Naruto Uzumaki (Naruto) made a contract with the toads, and can summon them by offering a drop of blood each time.
Ursula (The Little Mermaid) has a way of making deals with her victims.
Rokumon (Kyōkai no Rinne) is Rinne's black cat of contract. The black cat's have a symbiotic relationship with the shinigami, and assist them in their duties of casting souls into the underworld and placing them on the path to reincarnation.
Celestial Spirit Mages (Fairy Tail) make contracts with Celestial Spirits to summon them with Spirit Keys.
Kyubey (Puella Magi Madoka Magica) is a being known as an Incubator which is a being capable of giving contracts for the price of one's soul in return they get one wish.
The God of Death (Yu-Gi-Oh! GX) is an evil duel spirit who offered the wielder the power of one-turn kill in exchange of their souls.
Kuro (Kurokami/Black God) a Terra Guardian is empowered by her contractee/human's life force and their bond.
Contract (Oto x Maho) that allows a person to become a magical girl and this contract can only be given out by magical girls. Failure to follow the contract will result in the person to live as the opposite sex.
Hades (Hercules) was able to rob Hercules of his god-like strength through a contract, Hercules' strength returning when he was unable to keep his end of the bargain.
Crossroad Demons (Supernatural) operate by making contracts with humans, granting a human their wish in exchange for ownership of their soul.