Crystal Ball

"Palantir-phone. Great reception, and all weekend calls are free! Now with 50% less chance of eldritch evil mind-crush!"

"I don't need help, it's obvious what this means: there's going to be loads of fog tonight."

—Ron Weasley looking into a crystal ball, Harry Potter and the Prisoner of Azkaban

Common prop used by witches and psychics (and Hollywood Roma) for fortune telling, scrying, and long distance communication. Most commonly it's a dinner plate sized sphere of clear glass, but these can be of just about any size and material, so long as they're shiny.

Despite their ubiquity, there are many, many more ways to tell the future or view remote locations. Pools of water are mystic favorites, as are thrown bones, giblets, and of course Magic Mirrors. Nor should we forget the Magic 8-Ball.

Examples of Crystal Ball include:

Anime and Manga

Konoka and Anya in Mahou Sensei Negima sometimes use these (strange in Konoka's case, since she uses eastern-style fortune telling). Evangeline uses hers to communicate over a long distance like a phone.
In Final Fantasy Unlimited, Fabula is rarely seen without her trademark blue crystal ball. It shatters early into the second season, however.
The third Hokage in Naruto had a crystal ball which he used two times within the first two episodes, and never again.

Film

Jareth in Labyrinth keeps plenty of these on hand, watching Sarah through them and occasionally channeling his magic through one. He sometimes spins them in one hand; this is not, however, David Bowie doing these tricks. A professional juggler is standing right behind him and doing them blind.
The All-Seeing Eye in The Thief of Bagdad that shows Abu where to find Ahmad.
The Wizard of Oz has two crystals. Professor Marvel has a small one in which he claims to see Aunt Em searching for runaway Dorothy, crying and possibly having a heart attack. The Wicked Witch of the West has a much larger one in which Dorothy herself briefly glimpses her Aunt Em and tries to communicate with her.
Madam Estrella, the gypsy fortuneteller used one when she wasn't creating or commanding The Incredibly Strange Creatures Who Stopped Living And Became Mixed Up Zombies (of Mystery Science Theater 3000 fame), you feelthy peeg!

Literature

The Lord of the Rings has the Palantiri, a set of seeing stones that allow the viewer to scry far off locations and communicate between each other. Of course, some were destroyed, and one fell to Sauron, making their use incredibly dangerous considering his penchant for Mind Rape or Mind Control.
- Middle-Earth had a few other clairvoyance-granting artifacts. Galadriel's Mirror (actually a pool of water) could show things from the past, present, and possible future, and the seat at Amon Hen would allow one to see far-away present events.
- Parodied in Bored of the Rings with the Mallomar, "the magic watchamacallit of the elves." Moxie thinks it works like a Ouija board, but it's more like a fancy Magic 8-Ball.
Of all things Harry Potter seems to eschew this convention. Madam Trelawny spews prophecy regardless of props, and the crystal balls are pretty much useless except as bombardment.
- Interestingly, Pensieves fulfill this function. Snape uses Dumbledore's Pensieve to store memories he didn't want Harry to see yet. And the memories in the Pensieve (when not actually using it) are described as halfway between silvery water and gas.
Large shiral crystals are used this way in the Deryni works. Alaric Morgan has a fist-sized one in a golden gryphon holder in Deryni Checkmate, and the Camberian Council have a larger one suspended from the ceiling of their meeting chamber.
The windows of The House in The Redemption of Althalus serve this purpose.
Used both straight and subverted in Discworld novels:
- Wizards often complain they can't get good reception on them (ironically enough, using it to watch a rock concert in Soul Music). Witches and mediums such as Mrs. Cake only use them when paying customers hang around, but in the privacy of their own house they can use bits of glass or water or anything else they like.
- They used one in Going Postal, with this result:

Ridcully: It's not working, Mr. Stibbons! Here's that damn enormous fiery eye again!

Stephen King puts a novel twist on this trope in The Dark Tower, as the thirteen color-coded crystal balls of the Wizard's Rainbow are each a corrupting Artifact of Doom.
In The Riftwar Cycle, the first sign that Pug had potential as a magician came when he inadvertently saw something in a crystal ball.
Called a Gramerhain in the Bahzell Bahnakson Series, and far more limited than most. The victory in the third book probably wouldn't have happened if they'd been as omnipurposable as most crystal balls.
In Robert E. Howard's Conan the Barbarian story "A Witch Shall Be Born" the title witch holds this in comtempt as a means to study, but uses it for communications.

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.

In Gene Wolfe's The Sorcerer's House, the psychic has a fake crystal ball that sees good stuff and the real thing.
One of the talking magic items Aahz collects in Myth-Gotten Gains is a crystal ball, in which the face of a female of the observer's species manifests when it's communicating with people.
In Cerberon, these are called scryballs. They are used to view other locations, communicate with other scryballs like a telephone, or to spy through uncovered scry balls. Aladavan keeps a miniature scry ball on a chain around his neck, which he keeps under his shirt when he's not using it. He's able to easily spy on and track Thedrik because his sword has a small scryball on the pommel, which Thedrik never covers. Scryblocks are employed to prevent people from using scryballs to spy on them, and to protect against mental eavesdropping.

Live-Action TV

Battlestar Galactica had a prophetess in New Caprica use a bowl of water to divine the future for Number Three. Then there's the Cylon's Unusual User Interface, which consists of placing their hands in streams of water; and the prophetic Hybrids in the baseships, who are submerged in water.
The viewing globe in the original Mighty Morphin Power Rangers was one.
- Power Rangers Mystic Force also has one, able to detect dark magic anywhere in the world and reveal its source.
The fortune teller in The Monkees episode "Monkee See, Monkee Die" has one at the seance (& another one that's just a snowglobe).
Nimueh in Merlin can spy everything that goes on in Camelot with something like a font filled with water.
- Similarly, Morgause uses a crystal.

Music

Iron Maiden's "Can I Play With Madness?" is about a man going to a prophet with a crystal ball - in which, for the man himself "there's no vision there at all".
- In fact, Seventh Son of a Seventh Son (the album that song is on) is a concept album about a man blessed and/or cursed with prophetic sight by virtue of his being, well, the seventh son of a seventh son.

Video Games

There are several Crystal Balls in Loom, all of which are used for telling the future (and learning new drafts).
There is a sidequest involving a fortuneteller's crystal ball in Arcanum. The consequences of stealing one are... severe.
Nigel gets his fortune told in one in The Lost Crown: A Ghost-Hunting Adventure.
The casual game Adventures Of Robinson Crusoe includes a minigame where you must identify pictures in a crystal ball, using as small an area of the picture as possible.
One acts as a map in The Adventures of Robin Hood.

Web Comics

Xykon from Order of the Stick uses one to scry upon the heroes regularly. It's even widescreen.
Early on, in Adorable Desolation, Shopclerk peers into one, first he sees a vision of a skull and then the Crystal Ball cracks in half.
Dominic Deegan has numerous forms of crystal balls. Usually they're just for divination and scrying, but they have also been used similar to telephones. They also aren't just the conventional clear, either. Miranda Deegan uses one made out of onyx.
Homestuck has both Rose and Jade using crystal balls for remote viewing; Jade combines hers with a super computer so she can remote view all possible locations at once
In El Goonish Shive, this is used to frame a sketch in this filler comic.
In Impure Blood, they have timepieces that print out messages -- or ought to.

Western Animation

Madam Wu in Avatar: The Last Airbender heats bones and studies the cracks. Aang has such a huge destiny his bone explodes. When madam Wu excitedly tells him of his destiny to fight Fire Lord Ozai in order to restore balance to the world, he's more interested in if there's anything about his love life in the smoking splinters. (He already knew about Ozai, after all.)
In the ALF cartoon episode, "Housekeeping For Pokipsi," Madam Pokipsi the "Fortune Smeller" has a Magic Ball that is not only use for divination, but is the source of all her arcane powers; when Gordon messes with it, he ruins her magic tricks ("Vhat haf you done ma Crystal Bawwwwl, Yunk Mayun?").
In Thundercats 2011 the Sword of Omens is a hybrid of this and a Magic Mirror. Staring Through the Sword prompts the spherical cabochon Eye of Thundera in its hilt to send a vision visible in the Blade Reflection.

Real Life

Practitioners of ceremonial magic and some forms of witchcraft use crystal balls. Rather than expecting to see anything in them like a TV set, you are supposed to use them simply as a focus of attention, similar to meditation. Anything you see should appear in your mind, not in the crystal.