Chekhov's Gift

Dumbledore sent me a plot-point for Christmas.

Happy Birthday. What a wonderful day this is! You're a year older, so let's celebrate! Now, I know the mystic artifact I got you seems useless and thoughtless, but something told me it would be good for you. Yeah, see it glowing. It does that sometimes. Keep it in mind when you're fighting the Dark Lord. If you're lucky, it will actually be important to the plot.

Now, Chekhov's Gift is not given an important purpose at first. It gains that with time. Now, that old hermit might give a gift that he knows will be useful, but you do not know immediately how important it is to the plot. The other case is that it was important by freak chance. It can be given for a Birthday, Christmas, or some other reason. The key factor is that it's not what it appears to be at first. Sub-Trope of It Was a Gift. See also It May Help You on Your Quest. Compare Memento MacGuffin.

Enjoy your sacred artifact. Hope you manage to Save the World.

Related to the Law of Conservation of Detail.

Examples of Chekhov's Gift include:

Anime and Manga

In Kakashi Gaiden, Minato's gift to Kakashi seemed like an ordinary if oddly-shaped kunai, but it was actually meant to summon Minato to Kakashi's location once it was thrown.
In Pokémon Special, Blue gave Yellow a straw hat for disguise and gender-concealing reasons. It ultimately doesn't seem like it does much other than creating awkward tension between her and Red later, but it is eventually revealed that the two feathers that adorn the hat are really the Rainbow and Silver Wings.
The heroine of Nurse Angel Ririka SOS gets her Transformation Trinket as a birthday present.

Film

At the end of Star Trek II the Wrath of Khan, Kirk realized that the birthday present Spock gave had a secret message hidden in it.
- Though, to Kirk's credit, he did suspect something when Spock gave the gift.
In Clash of the Titans, Zeus gives three gifts to Perseus: a sword, a shield and a helm of invisibility. Later, Athena gives him Bubo the owl. All of them are useful to Perseus in his adventures, one way or another.
Name a James Bond film! Bond always receives plenty of presents from Q. Some of the presents are made specifically for Bond's mission, but most really aren't. Coincidentally, and conveniently, every function of every item from Q just happens to be useful once (and only once) on Bond's mission.
- Most specifically, Licence to Kill, in which Della and Felix Leiter give Bond a cigarette lighter as a wedding present. It is later used to light an petrol/cocaine soaked Franz Sanchez on fire, sending him to his doom.
- There is one exception - the exploding alarm clock from License to Kill ("Guaranteed to not wake up anyone who uses it") was never actually used.
The British Horror Film "Dog Soldiers" features this.
This is actually subverted in the first Star Wars: A New Hope film; at the beginning it makes a fairly big deal about Luke being given his father's lightsaber by The Obi-Wan, but Luke never ends up actually using it for anything useful, and in the end saves the day with the piloting skills he picked up as a farmboy.
- Of course it does prove handy in the second film, saving Luke's life on Hoth.

Literature

Harry Potter gets a lot of these: his invisibility cloak, a flute, his firebolt, and everything Dumbledore left Harry in his will.
- Everything Dumbledore left Ron and Hermione in his will as well.
- Hermione's cat, Crookshanks, was an early birthday present that was crucial to the plot of the third book.
- Zigzagging with Sirius' magic knife. After he received it, he forgot it and just remembered that it would have been useful while in the second task of the Triwizard Tournament. In the next book he uses it once to break into Umbridge's office. Later, he remembers to take the knife with him to the ministry, but then it's melted and rendered unusable when he tries to open a locked door.
- Sirius' magic mirrors. Instead of using them, Harry uses the fireplace to communicate with him and gets himself caught by Umbridge, as well as getting fooled by Kreacher, resulting in Sirius' death. Harry only remembers about them later and makes an attempt to communicate with Sirius after he's already dead, which obviously doesn't work. The mirrors are brought back into play in the seventh book when Aberforth Dumbledore is using the other one to keep track of Harry.
Father Christmas' gifts to the Pevensees in The Lion, the Witch and the Wardrobe.
The One Ring, perhaps?
The Curse of Chalion: Lord Dondo gives Royesse Iselle a strand of pearls twice as long as she is tall. Much later, they are instrumental in saving the life of her betrothed.
In A Game of Thrones, Daenerys gets a set of three stone dragon eggs as a gift for her marriage to Khal Drogo. Turns out at the end of the book that they're not just rocks after all.
In The Dresden Files Bianca gives Lea (Harry's very scary, very powerful and not entirely sane to begin with) faerie godmother an athame which is said to be in the same power league as Amoracchius, which is implied to be the most powerful/untinkered with, of the Swords of the Cross. The next book, Lea passes on Harry's debt to her to Mab so she can keep the blade, which drives her batshit insane. Then Mab, Queen of the Winter Sidhe and on a par with gods and lower Archangels, gets hold of it. Cue massive Oh Crap moment.
In Cerberon, Aladavan, a sidhe wizard, gives Robert a special sword called Magister, which later becomes important in Darkram's Secret Test of Character. It may have other significance that isn't revealed in the rest of the novel. (Maybe the next one?)

Live-Action TV

In Monk, Trudy gives Monk a Christmas present just before she dies. Monk has refused to open it for 12 years. When he finally does open it, it's a videotape of herself telling Monk who her killer might be.

Video Games

A less superpowered example is the voice-recording pendant that Kana: Little Sister's heroine receives from her brother in the opening scene of the game. She uses it to record her final message to him, which he listens to in the end after Kana has died of organ failure.
In the retro point-and-click adventure "simon the sorcerer" on Simon's birthday a Dog arrived at his house carrying a Book adorned with unreadable symbols (It later turns out to have been calypso who was giving these to simon so he could rescue him). He shoved the book in the attic, thinking it worthless, and adopts the dog as a family pet. Later on the dog goes into the attic grabs the book and gives it to simon, who to his astonishment, can now read the book, but still thinks it's garbage and throws it over his shoulder. This triggers a portal to open to the magic kingdom where his adventure begins. Most of this occurs in the back-story (which is in the manual) and doesn't appear in the game
In Mother 3 the Courage Badge Flint gives to Lucas (through Nippolyte) turns out to be a Franklin Badge.
In the Visual Novel Katawa Shoujo Hisao goes into an Antiques store in order by a present for Hanako's birthday in Lilly's route. While he is there he notices a music box and decides that Lilly would like and purchases it as a gift for her as well in her Good Ending she plays it for him in his hospital room to let him know that she didn't get on the plane and his staying in Japan with him.
Midway through Castlevania Dawn of Sorrow, Soma gets a talisman from Mina. It's a useless accessory, but you need to equip it to avoid getting the Bad Ending.

Web Comic

In Homestuck, John receives birthday presents from his friends. Because of Stable Time Loops, the presents turn out to all be the authentic stuffed bunny from Con Air, at different points throughout its history. The final version of the bunny ends up saving John's life when he is cornered by the Big Bad.

Web Original

Whateley Universe example: in one Chaka story, Chaka's ring gets coated with mithril (there's a mithril-secreting mutant at Whateley too) so she can't wear the ring. The mutant gives her a mithril bracelet, and Chaka gives the ring to Generator as a gift, in part because Generator is the only one small enough to wear the ring now. Both of these Chekhov's Gifts turn out to be lifesavers in later stories.

Western Animation

Anti-example from Winx Club S4: The Winx receives the gift of life from the ethereal fairies in ep 23. Just the thing for saving Nabu when he's fatally injured during the Black Circle's betrayal of the earth fairies in the next episode...that is, until the Black Circle also take that away from them.
In "Secret of My Excess" in My Little Pony: Friendship Is Magic, the fire ruby Spike gives to Rarity.
In an episode of The Simpsons, Bart gives Lisa a laser pointer as a birthday present. Later, Lisa uses that laser to blind the villain who is trying to kill Homer