Magic items are given to Greek and Roman demigods by their godly parents. These items can be used by the campers for many purposes. One of the more common places they are used is in the game of Capture the Flag and War Games but they also come in handy for fighting monsters.
Percy's Magic Items
Riptide, also known as Anaklusmos, was given to Percy Jackson by Chiron in The Lightning Thief to fight against his "math teacher" Mrs. Dodds (who was a Fury disguised as a human). It is made out of Celestial Bronze which does not harm mortals but "passes through them like an illusion". It has the magic ability to never get lost. If thrown away, it will appear in his pocket a few moments later.
Percy receives his shield from Tyson, a Cyclops who is his half-brother. Tyson made it himself at Camp Half Blood. It transforms from a watch into a shield. It is dented in The Titan's Curse, repaired by Tyson, then lost in The Battle of the Labyrinth.
As Percy is leaving on the quest in The Sea of Monsters, he sees Hermes who gives him some gifts.
Hermes gives Percy vitamins that counteracts all magic for a time. Percy and Annabeth uses them to escape from being a guinea pig after being turned into one by Circe. There are different flavors, and are in the shapes of the Minotaur, a Fury, and other things.
Hermes gives Percy a thermos that contains the four winds of the North, South, East, and West. The lid is also a compass. The cool side always faces the north. Percy lets one wind go to gain speed to catch up with Clarisse La Rue.
Annabeth's Magic Items
Annabeth has a magic Yankees baseball cap that, when put on, makes the user invisible. It was given to her by her mother, Athena, for her twelfth birthday. Percy and Annabeth are the only one's that use it. After Annabeth was confronted by her mother in The Mark of Athena, it stopped working. As of The Staff of Serapis, the cap is refunctioning.
A coin, known as the 'Mark of Athena', has symbols of Athena on it and leads chosen demigod children of Athena to the Athena Parthenos.
In his first game of Capture the Flag, Percy was told to watch out for Clarisse's spear. He later got hit by it and felt an electric shock. When he fought back, he broke the spear causing Clarisse to become very angry. In The Last Olympian, she gets a new spear which she calls "Maimer," but which everyone else calls "Lamer" behind her back. It was broken when Clarisse charged the Drakon during the Battle of Manhattan.
Luke's Magic Items
Luke has winged shoes given to him by Hermes. He gives them to Percy in The Lightning Thief, but they were a trick that tried to drag him into Tartarus. Percy gave Grover the shoes and they tried to take him there instead. The wings will sprout at the heels when the word "Maia" is shouted.
The Hunters' Magic Arrows
These arrows explode into a cloud of stinky yellow smoke when they hit something. They are used by the hunters in the game of capture the flag and Thalia Grace calls them "unsportsmanlike".
Apollo's Cabin's Arrows
Sonic arrows make a screaming noise, when they are shot. They are also given to the Apollo's cabin members as gifts during the Battle of Manhattan.
Nico's Magic Items
Stygian Iron Sword
Stygian Iron is very rare, since it must be cooled in the River Styx. It is also impossible for a non-child of Hades to be able to wield it. The sword channels Nico's power. In The Battle of the Labyrinth, Nico pointed his sword and said "Serve me" to summon a legion of dead. It would probably have been much harder, or much more tiring to summon all those dead without his sword. Stygian Iron, unlike Celestial Bronze can kill both monsters and mortals. Celestial Bronze turns them into dust, while Stygian Iron sucks away their soul and essence. Because the essence is absorbed, the monsters will die and will not reform unless the weapon is destroyed.
Thalia's Magic Items
Aegis is Thalia's magical shield, a replica of the real Aegis, owned by Zeus and Athena, and has Medusa's head imprinted on it. Monster and demigods alike are known to run from the sight of it, because it radiates fear and terror. So strong is its aura that monsters cower at the sight of it. Thalia uses it in battle, along with her Hunter weapons and her collapsible Mace canister spear. She found it inside a locked chest in a mansion during the time she was on the run with Luke Castellan, and it has saved her life many times. The code word to activating it is 'Aegis', which Luke found out while inside Halcyon Green's mansion. When not activated, it looks like a silver charm bracelet, which Thalia wears on her wrist.
Jason's Magic Items
Ivlivs is a gold coin belonging to Jason Grace that, when flipped, turns into a Double Edged Sword or a Lance depending on which side the coin lands on. The Sword side has an image of Julius Caesar, while the Lance side has an axe and the word Ivlivs. It is made from Imperial Gold. He first learns that it could turn into a Lance when showing the other campers that he is a son of Zeus by calling down lightning. It was later destroyed in a fight against Enceladus.
Piper's Magic ItemsPiper McLean's dagger and her primary weapon. Piper finds and chooses it when Annabeth takes her to the Armory. It is sheathed in worn, black leather and is bound in bronze. Katoptris has a hilt made of polished wood with an 18" triangular blade made from Celestial bronze. It is useful in battle and is a symbol of power and status. This knife was originally a wedding present to Helen of Troy from her husband Menelaus, and when looked upon, can show much more than the holder's reflection (as shown when Piper saw her father tied to a pole by the giant, Enceladus).
Leo's Magic Items
Leo found the toolbelt in Bunker 9. Anything that can be found in the average workshop can be pulled out of the toolbelt. The belt is seemingly endless and he doesn't know where it all comes from. However, larger items, especially things that don't have to do with mechanics require the belt to recharge. It is also known that big items, such as chain-saws, or weapons, like Jason's sword, Ivlivs, cannot be pulled out of the belt, because they aren't really known to be found in a workshop. It is implied but currently untested, that Leo is able to put items into the belt and retrieve them at command (items that did not originate, as parts or the full item).