"Fillet of a fenny snake, In the cauldron boil and bake; Eye of newt and toe of frog, Wool of bat and tongue of dog, Adder's fork and blind-worm's sting, Lizard's leg and howlet's wing, For a charm of powerful trouble, Like a hell-broth boil and bubble."
The material (or immaterial) component you need to call forth a spell or activate a superpower. It's not as simple as just spending Mana, though. You may need to burn a pinch of sulphur, or to sacrifice the soul of your first born child. Either way, you've got to pay the price before you can throw lightning from your fingertips. If the value of what's sacrificed has to equal the value of what's gained, it's Equivalent Exchange.
These components usually fall into one of four categories, with some overlap.
Symbolic items: wedding rings, grave dirt, a pure red rose. Some materials, such as gold and silver, carry heavy symbolic value all by themselves.
Body parts from exotic creatures, sometimes from creatures that don't actually have those body parts - hen's teeth. The more magical the creature the better. A unicorn's horn beats a crocodile's liver, but the blood of a god is better still. Doubly so if it has crystallized. Upping the danger, the creature is sometimes a Monster Lord.
Items related to (or body parts from) your intended victim or other recipient such as articles of clothing, treasured trinkets, personal affects, hair, baby teeth, fingernail clippings, fingers, urine samples, blood etc.
Items with improbably specific requirements - an unripe Sunset Wonder picked 3 minutes before noon on the first frosty day in the autumn and peeled left-handedly using a silver knife with a blade less than half an inch wide.
In the Ranma ½ manga sometimes spell components are needed by the magic users, for example, Happosai needed the tears of a creature both male and female for a rejuvenation potion.
Happens once or twice in the anime, too. In "The Last Days of Happosai...?", Akane Tendo tries to prepare a magical elixir that will revitalize the dying pervert- it's implied her usual lack of skill in the kitchen is the source of the potion's nauseating stink, which eventually renders all of the others in the house bedridden with sickness. Eye of newt and toe of frog are even actual ingredients.
In Great Teacher Onizuka, one member of a trio of witch-wannabes drinks a Love Potion and accidentally sees Onizuka first. They try to use black magic to negate the power of the spell. Ingredients include bat wings (procured from the science department), toad warts (ditto), and Onizuka's pubic hair (uh...).
All Getafix will reveal to Astérix about the ingredients of the magic potion is that it contains mistletoe harvested with a golden sickle and lobster. The lobster is optional, but it improves the flavour. A few other ingredients are revealed through plot contrivance - such as a tiny drop of "rock oil" (petroleum), although through research it is determined that it can be substituted with beetroot juice which has the exact same effect, is easier to get and doesn't taste as bad, and "reasonably fresh fish". It's implied that in so far as its chemistry goes it's really just a mediocre vegetable soup, albeit with added magic effects. Several times its obscure ingredients are used as Plot Coupons, such as in The Black Gold, The Great Crossing, and Asterix in Switzerland (although that was for an ingredient needed to brew an antidote for a poison).
Parodied in the movie Robin Hood: Men in Tights, where Latrine is apparently putting together a scrying spell with all sorts of gooey ingredients, including "eyeballs of a crocodile". A moment later we learn she's not a witch at all; she's the cook.
Warlock used the body fat of a non-baptised child as a levitation potion. Baptise your children, people!
We often see witch spells requiring ingredients, such as in Wyrd Sisters, a direct sendup of Macbeth which contains a scene parodying the above quote. Granny Weatherwax, for example, complains about using up a "tiger's chaudron" that "looks like perfectly good chitterlin's ... there's hungry children in Klatch who wouldn't turn up their noses at it." And tongue of boot.
Furthermore, the cottage Magrat lives in used to belong to a "research witch", who asked questions like "it's all very nice to say 'eye of newt', but what species of newt? And would it still work if you substituted something less icky?" and wrote all her research down in dozens of volumes. (Turns out it works, though: Magrat later uses one such carefully defined spell to find out her boyfriend's first name. She starts working on a love spell too, but plot intervenes before she completes it.)
In the Young Wizards series many spells used to require hard to find physical components, but as successive generations of wizards improved the spells the components were changed to easier to find substitutes, and eventually the spells were perfected to the point where they needed no components at all. The modern-day characters which the series follows only rarely have to cast a spell which requires any sort of physical component.
In the book The Princess Bride we are told that they had to search for strange components before Miracle Max could do a miracle, but we aren't shown it because it would take too long.
Ingredients in Harry Potter potions include a bezoar and bicorn horn, and the brewing of Polyjuice Potion involved particular parts of a lunar cycle.
The spell that resurrected Voldemort in Harry Potter and the Goblet of Fire required "bone of the father, unwillingly given" (Voldemort's father's bone, stolen from the grave), "flesh of the servant, willingly sacrificed" (Wormtail cut off his own hand), and "blood of the enemy, forcibly taken" (cut from Harry's cheek).
The Dresden Files sometimes uses non-physical components gathered under specific conditions. E.g Harry had to be truly happy in order to gather sunlight into a handkerchief.
Potions specifically need 8 ingredients. A base liquid, one for each sense as well as the spirit and the mind. A love potion, for example used tequila as a base, money for the mind, chocolate for taste, perfume for smell, lace for touch, a sigh for sound, candlelight for sight, and the ashes of a romance novel for spirit (though it probably would have worked a little less sleazily if they hadn't used substitutes for the original base, mind and spirit ingredients - champagne, powdered diamond and the ashes of a love letter.)
Ordinary magic can be done without physical ingredients or foci, but no-one does it that way. You can just create the things you need in your mind, but if your mental image slips just a little, your spell will fail. Trying to do it that way, rather than with a physical object, is much more difficult and makes no difference in effect, so no-one bothers unless the midden hath hit the windmill, big time.
In The Legends of Ethshar series, Ethsharian wizardry uses ingredient like this - a raindrop caught in midair, the blood of an unborn child.
One of the characters in Iron Council is a monk from a special order that discovers secrets. To do so, however, the monk has to sacrifice one of his/her own memories or abilities each time he digs up new info.
In The Neverending Story, Bastian discovers that every time he uses his amulet to "change" things, he sacrifices one of his own memories. Eventually, he develops full amnesia. He gets better.
In Dream of the Red Chamber, Precious Virtue's Cold Perfume Pill has a vast list of peculiar ingredients which are so rare they can only make a batch every twenty years or something.
In the Enchanted Forest Chronicles, Cimorene searches in vain for months to find some hen's teeth so that she can complete a spell to protect her against fire (she's employed by dragons). She eventually has to get them from a genie.
A couple of books from Kushiel's Legacy use this trope; the end of the first series ('Kushiel's Avatar' I believe) has the bone-priests that only get their power by sacrificing someone that they love. The Mharkagir tries this with Phedre but she kills him instead; and there was much rejoicing.. 'Kushiel's Mercy' (end of the second series) has Carthage trying to take over Terre D'Ange with some pretty involved magic. The stone trapping the elemental has some pretty icky requirements ( infanticide being the big one) and the needle that afflicts Imriel with madness (and thus saves him from the bigger spell the Carthaginians are casting) requires toad-bile, lunatic sweat and being left in the light of the full moon (and NOT being in any other light) for a full month. Wonder what the process was for finding all that out.
I could never endure to seclude myself in a golden tower, and spend the long hours staring into a crystal globe, mumbling over incantations written on serpent's skin in the blood of virgins, poring over musty volumes in forgotten languages.
All discarded portions of the human body still remain part of it, attached to it by intangible connections. The priests of Asura have a dim inkling of this truth, and so all nail trimmings, hair and other waste products of the persons of the royal family are carefully reduced to ashes and the ashes hidden. But at the urgent entreaty of the princess of Khosala, who loved Bhunda Chand vainly, he gave her a lock of his long black hair as a token of remembrance. When my masters decided upon his doom, the lock, in its golden, jewel-encrusted case, was stolen from under her pillow while she slept, and another substituted, so like the first that she never knew the difference. Then the genuine lock travelled by camel caravan up the long, long road to Peshkhauri, thence up the Zhaibar Pass, until it reached the hands of those for whom it was intended.
In The Patchwork Girl of Oz, one of L. Frank Baum's Oz books, Ojo is collecting the ingredients to restore people from statues. He is arrested for collecting a six-leafed clover; Ozma made it illegal to collect such ingredients because people refused to obey her anti-magic law. Later, he finds the hardest — a drop of oil from a living man — which is from the Tin Woodman. Alas, he also needs the left wing of a yellow butterfly, and the Tin Woodman refuses to allow a butterfly to be harmed for the spell. Luckily, Glinda the Good doesn't need these ingredients.
In L. Jagi Lamplighter's Prospero's Daughter trilogy, phoenix lamps, lit by phoenix feathers, and the Water of Life, retrieved from a well at the edge of world, are the first of many, many, many such items.
In War of the Dreaming by John C. Wright, magicians use symbolic objects to compel obedience from the spirits who respond to them—such as moon rocks from the Apollo missions.
Played for comedy in A. Glushanovskiy's Road to Mage, first novel of the Way of the Demon series. Oleg, the protagonist, found a spell to detect precious metals underground and already thought of using substitutes for a "dragon's vertebra" - a dinosaur's vertebra from a museum, and for "river-horse hair" - a hair of a tame hippopotamus from the zoo. But he's out of ideas to obtain or substitute "lock of hair from a chaste actress", "spit of a truthful lawyer" and "blood of a honest secretary of state".
Obligatory Buffy mention. Various ritual spells require various components, some even require the Eye of Newt. Although according to the resident witch, eye of frog is cheaper. Really, you're just paying for the brand name.
In Angel Wesley is analysing a Fantastic Drug which has PCP-like effects on demons, and mentions that Eye of Newt has been added to improve the taste rather than the kick.
In Charmed, potions sometimes require these, but good ones usually use more benign herbs.
In Supernatural, many rituals and spells require a list of items, such as the photo of the summoner, graveyard dirt, yarrow, and bone of a black cat, required to summon a crossroad demon.
Stormwitch song "Stronger Than Heaven" begins with a listing of various magical ingredients.
Mandrakes, three black feathers
Dried up toads and rats
Spiders, human leather
Eyes and wings of bats
A number of spells in many versions of Dungeons & Dragons (AD&D 1e, AD&D 2e, and D&D Third Edition) require use of material components. For standard spells, like fireball, this requires something trivial and commonplace (like bat guano and sulfur rolled into a ball) that one can BS away by having a spell pouch on them. For more powerful spells, like Raise Dead, you're expected to pay cash money to use them (in the form of a pile of diamonds worth 5000 gp).
Bat guano as seen in thisOrder of the Stick webcomic (see the image at the top of this page). 5000 gp worth of diamonds mentioned in this one.
Toyed with in this one, with the valuation being crucial to the spell.
Still, if that sort of thing isn't easy enough for you, there are feats like Eschew Materials (which is Exactly What It Says on the Tin, for material components that don't mention a money cost), prestige classes like the runecaster, which allow you to replace expended material components with permanent rune-carved objects (one wonders what the replacement for fireball is...a little stone ball with "bat poo and sulfur" carved in Draconic?), and others.
D&D Fourth Edition has removed this from standard spells, but the more powerful rituals require material components.
Similarly, material components, while not needed for most spells in Warhammer Fantasy Roleplay, did provide a (generally trivial but) much appreciated bonus to your spellcasting attempt.
In Mage: The Awakening extended spellcastings are made easier by the sacrifice of a 'sacrament'; an item metaphorically relevant to the spell being cast (for example, burning a map to create a portal).
Also, for Archmasters to cast an Imperial (i.e. godlike) spell, they require a 'Quintessence', a metaphorical component, such as an ingredient or event.
"Gross matter", substances which can be imprinted with magic (effectively making potions) can only be manufactured by gathering materials thematically relevant for the kind of gross matter you are trying to make (different kinds can be imprinted with different spells; for example, spells that affect perception need to be imprinted into eye drops), before using a spell to transform it.
In Urban Arcana, the modern day Dungeon Punk variant for d20 Modern, a special variant of spellcasting called "Incantations" are available. These Incantations are lengthy yet powerful procedures that require materials appropriate to the spell in question. For instance, demon summoning would likely require a virgin sacrifice and an obsidian knife, whereas the consecration of a building would require holy water and a recitation of prayer.
Also, ordinary spells might still require material components, such as a can of soda to make people go faster and the CTRL ALT and DEL keys from a keybord to shut down any electric device in a given radius.
Ritual magick in Unknown Armies has this as its great drawback. Rituals may need anything from a scratched brass doorknob to your own eyeball to "acres and acres of burning tires".
GURPS Magic mostly handwaves these away: most spells require some sort of unspecified material components, but wizards usually have what they need on hand. The Game Master is advised to elaborate on this if a shortage of a component would help curb abuse of a problematic spell, or just provide plot hooks. GURPS Thaumatology provides optional elaboration on this, and its alternate magic systems go into detail about the use of material components in folklore and fiction.
In Changeling: The Lost, hedgespun items are created in and using components taken from the Hedge. These can include the relatively simple, such as vines or shells, or the complex, such as refined metals, but every ingredient needs to have some sort of story behind it, one through which the players take their characters in the process of gathering said components.
The human bits are pretty specific. Ethnicity often matters; in one case the age, parentage, and cause of death matter; and in another the attitude in which the person died.
The Witch in Into the Woods says she can lift the curse on the Baker if he brings her several items: the cow as white as milk, the cape as red as blood, the hair as yellow as corn, and the slipper as pure as gold. Each of these items, of course, comes from one of the other fairy tales being told.
The Elder Scrolls also has a whole alchemy system that allows you to use ingredients with set effects to make potions with those effects. Or you can just eat them and get their effects for a brief time, even though some of the ingredients are plainly inedible. (Raw Glass, anyone?)
You still need to have a high enough alchemy skill just to get all the effects of eating it directly. It's Hand Waved that it has to do with a specific way you chew it.
According to Ashley's theme, an eye of newt is a component of one of her favorite hexes.
And Grandma's wig and kitten's spit!
Ninjas in Final Fantasy XI require ninja tools to perform their ninjutsu spells. The sheer amount of tools used to tank (No, Really), as well as the cost of the other tools make Ninja one of the most expensive jobs in the game.
Likewise, Corsairs need elemental cards to fire elemental blasts from their guns.
The first three Ultima games had rituals with elaborate requirements for each spell, but they were All There in the Manual. Ultima IV had you manually mixing up spells out of each reagent, typing incantations in the game's Fictionary, and then binding them with a small sacrifice of mana for later casting (with more mana.) Later games kept the reagent system, but did the rituals for you automatically.
This is how magic works in The Sims 2. Spells are fueled by objects called reagents that you can either buy or make for free, though making them takes time. Good spells are made with good reagents, such as dragon scales willingly given by an elder dragon, and evil spells are fueled by evil reagents, such as literal Eye of Newt made by..well take a guess. This is Informed Ability and All There in the Manual, all the sim actually does is stir a cauldron to create the reagents.
In King's Quest III, Gwydion has to gather the ingredients from throughout the realm and use them to cast spells, all while fearing that his wizard master may return and smite him. This worked out nicely in the context of an adventure game, where manipulating items is always a core ingredient.
World of Warcraft. All you mages know exactly what I'm talking about. "What? You told me to port to Stormwind! Bah, alright, I'll come to Darnassus, but you're gonna pay for the reagents! This costs money, ya know!"
All caster classes have a few spells that require physical components; mages' portals are simply the most well-known. Most other ones can bypass this if the character has the appropriate Glyph.
All of the magic in Secret of Evermore revolves around alchemy formulas which each require different regents.
In Albion, one of your party's spellcasters requires a special seed to be thrown at target. You buy these or pick from bushes occurring occasionally in wilderness.
A potion in A Vampyre Story requires a nightshade blossom, a gargoyle's breath, a virgin's bone, and a literal eye of newt. The first three she manages to scrounge up, but it's the middle of winter, meaning no newts. So she uses the eye from a picture of a newt in a coloring book. This works perfectly.
RuneScape has lots of odd ingredients for spells, though baby blue dragon scales, limpwurt roots, white berries, snape grass, red spiders' eggs, and (what else?) the eye of newt are some of the body parts necessary for potions. Then you get the Rag and Bone Man, who asks for some really strange bones. Shoulder bone of a of giant? Tail bones from nine kinds of dragon? Pelvis of a four-legged, magic-casting water creature that dwells in caves? Fibula bone of the third leg on an adult three-legged creature? He wants them all and more. He serves an Eldritch Abomination that wants to rebuild itself with all the bones. Squick.
In Sluggy Freelance some spells from the Book of E-Ville require certain physical components.
The dark magic that Hekate does in the Whateley Universe, like her spell in "It's All In The Timing", is exactly this trope.
This picture by Helle Jorgensen reminds witches: take necessary steps to prevent possible contamination by unintended components.
Some magic in Phaeton requires this, also in a literal sense of the words, Teliha finds Eye of Newt to be a tasty snack and often eats it all before it can be used for magic.
Taken to a rather dark extreme in Friendship is Witchcraft with "Pinkie's Brew"; the "eye of a Newt" turns out to be from the foal Newt Pippington British-hooves.
In the universe of Tales From My D&D Campaign, any long-range teleportation spell requires an "eldritch eye", the eye of a powerful abomination.
An episode of Jackie Chan Adventures dealing with a Jiangshi had what's probably a parody of the third type - to permanently banish the hopping corpse, Jackie and colleagues were required to take a toadstool from a graveyard, place it in the Jiangshi's own left sock (which, of course, it wasn't about to just hand them), and throw the sock into a river.
That's not a parody, people actually believed that, though usually the sock was filled with rocks or soil from the vampire's grave. And yes, Chinese vampires hop.
A lot of Uncle's spells in general follow this theme where certain items are needed. The animal location spells each required an item going along with that animal.
There's also those dead blow fish and lizards he uses quite often.
"You always want more Eye of Newt. If it were up to you, the soup would be nothing but Newt Eyes!"
In the Donald Duck cartoon "Trick or Treat" Huey, Dewey and Louie enlist the help of a witch to get back at Donald for pranking them. She whips up a potion with quotations from Macbeth, and partway through she leans over to the nephews and says "This is the real stuff, you know. Right out of Shakespeare."
Hoodoo folk magic is made of this trope. Components can include (but are by no means limited to) red brick dust, graveyard dirt (perferrably from the grave of a soldier or a child. Or, for best results, aChild Soldier), coffin nails, dried bat hearts, and raccoon penis bones, not to mention various bodily fluids. Much of the lore comes down from rural areas in the 1930's, when such ingredients were much easier to obtain than they would be today (or not: a random Google search can and will turn up various shops selling such items online, many disturbingly authentic).