In One Piece, CP9 are masters of Rokushiki, one form of which is Shigan (Literally "Finger Gun") where they use this style of finger pose, but instead of firing anything, they stab people with their fingers, which become as powerful as bullets.
Boa Hancock's Pistol Kiss technique allows her to blow a kiss from her fingertip like a bullet. She has a large-scale variation of this technique called the SlaveArrow.
In Trigun, Vash the Stampede uses his fingers to pretend that it's a gun in order to threaten the Villain of the Episode, and then "shoots" it off once the Mexican Standoff has ended. Though it should be noted he hid his finger under his cloths, making it look like a real concealed gun.
In the manga version of Yu-Gi-Oh!, Bandit Keith mimes Russian Roulette in front of Joey in this way as a way of referencing just how crappy he's seen his life get. Later on, when he threatens Pegasus at gunpoint, Pegasus smacks him with a Penalty Game which has him mime Russian Roulette again, and lose. And then he dies, because Your Mind Makes It Real.
YuYu Hakusho gives us a non-comedic version of the "gun actually fires" variant: The Spirit Gun.
Several characters do the regular version as well, including a particularly memorable one from Keiko.
Wild ARMs Twilight Venom: This is done twice in the first episode.
First time it's subverted Loretta and Mirabelle to Kiel, using a bottle with cloth over it. Although Kiel looks around and strikes up a semi-friendly conversation with them.
At the end it's played straight, Cheyenne (even though he has his super-powerful ARM) uses his finger against the warden of the prison he was stuck in. Since the prison is a floating island (using magnetic rocks), this makes the Warden have his fear of heights take over, and end up falling off the cliff.
Later, the Second Mizukage uses this as a technique, which actually fires a piercing water bullet. He says the technique is a signature of the Hozuki clan, which Suigetsu is from, retroactively implying he was either preparing to use or at least referencing it in the aforementioned scene.
Gauron does this in Full Metal Panic! with his Humongous Mecha, which actually produces a Wave Motion Gun effect. It helps that the machine is equipped with the Lambda Driver, which creates a powerful energy field directed by the pilot's imagination, thus the Finger Gun gesture served as a focus.
Hild from Ah! My Goddess does a variant of this in one chapter of the manga, she puts her finger to her head in a finger gun shape and shouts "bang!" and collapses. She does this in order to extract a crystalized fragment of herself out of her body.
Chizuru from Yumekui Merry does the finger gun a few times, saying "bang" while doing it.
Mana in Mahou Sensei Negima! seems to be able to shoot small bursts of spiritual energy in this fashion (for situations when she somehow can't reach her handguns quickly enough).
Star Platinum from Jo Jos Bizarre Adventure can extend its index finger to long distances with bullet-like speed and tends to make this pose when doing so.
Johnny Joestar from Part 7 is a bit more literal than most examples, since his finger gun can actually shoot projectiles; namely, his fingernails.
This is Lamba's primary weapon in Dangaioh. The Dangaioh itself lacks this but gains a Power Palm through her instead.
Bakemonogatari: Meme Oshino does this once, putting his fingers to his head in the shape of a gun.
Sieglinde Jeremiah of Magical Girl Lyrical Nanoha ViVid is able to fire quick bursts of energy by making this hand gesture. It doesn't do much damage, but it could disrupt an opponent's attack when aimed at their face as Einhart found out when she attempted to do a dodge and counter.
Jensen threatens a security guard with his fingers, claiming to be "dangerously telekinetic". Of course, none of the guards believe him, until Jensen says "bang" and the guard falls over. The rest of the security force surrenders and Jensen thanks Cougar, his sniper backup in the building behind him.
A What If? comic, in which Spider-Man became an assassin, had him installing guns into his web-shooters; naturally, they were triggered with this pose.
Crank doubly subverts the "it actually fires" aspect: Chev Chelios breaks the standoff by miming shooting one of the henchmen, who proceeds to fall over dead with a bullet in his head. It's later revealed to be a shot from some other people who are also enemies of the Big Bad.
It receives a callback in Crank High Voltage, with a man who Chev Chelios was interrogating at finger-point suddenly develops a bullet wound to the head. Chev then examines his finger with some surprise, as though it were an actual gun.
In Gran Torino, Clint Eastwood's character does this to a passing car
In the film Serendipity, the bored sales clerk played by Eugene Levy does this in the warehouse. The director's commentary explains that this is quite out of the ordinary for a bored sales clerk, so he'd want to inject a little drama into the situation.
In Star Trek, Kirk "fires" his gun in sync to the firing of proton torpedoes during the Kobayashi Maru.
In Time Bandits, when Og is trying to escape from Evil, Evil uses his hand as an actual Finger Gun, pulling back on his thumb and forefinger as if they were the hammer of a gun and firing transformation blasts to turn Og into a half-pig.
Mr. Bean does this in his first movie, Bean. At the airport. The security guards were not amused.
The tie-in book Mr. Bean's Scrapbook gives his side of the story: "I was arrested for the illegal possession of an imaginary gun. I don't know what all the fuss is about, it only fires imaginary bullets!"
In Busty Cops due to the lack of a budget during the final showdown a cop had to go gunless and had to hold her hands as a gun. There's an incredibly fake spark when it fires.
"Shooter" McGavin in Happy Gilmore fires his finger at the hole when he sinks a ball as his douchebag calling card.
In Blackbeard's Ghost, the titular ghost has a physical presence, but is invisible to everyone except the protagonist. When he uses Finger Gun on some criminals, Blackbeard knocks them out, creating the impression that it actually worked.
In The Losers, Jensen eludes a trio of guards by pretending to have telekinetic powers, which he demonstrates by taking out two of the guards with his Finger Gun. The guards didn't know that his pal Cougar was watching.
Scott Pilgrim vs. the World has Kim fake suicide when viewing especially stupid things by pointing a finger gun at her head, making a "Blam!" sound, and falling over.
Also from Scott Pilgrim: the literal hand guns of the vegan police.
Paul Kersey does this at the thugs at end of the first Death Wish movie in a freeze frame shot, signifying that his days as "Mr. Vigilante" are not yet over.
In Chronicle Andrew uses his telekineses to knock down a hoodlum, punctuating it by pointing his finger and firing.
At the end of climactic scene in Taxi Driver, Travis Bickle pretends to be shooting himself with a Finger Gun when cops arrive.
In The Expendables: Exaggerated in one scene of the sequel, when Barney treats his Finger Gun as if it were actually his revolver, complete with cocking the 'hammer'; subverted when all the baddies he 'fires' it at fall over dead as though he actually shot them; then double subverted when it turns out all the bullets were actually fired by Friendly Sniper Billy the Kid.
In Legion a cop does this while paroling the streets of L.A.
In The Brothers Bloom, Stephen makes a finger gun at Bang Bang, who mimes the bullet impact in her forehead and flicks the hanging bulb behind her, as if the imaginary bullet had passed through her skull.
Jay 'Popinjay' Ackroyd in the Wild Cards series has the power to teleport other people or objects (but not himself). Normally, for his power to work he use his hand as a Finger Gun and 'shoot' at his target.
An episode of The Office ended with a Mexican Standoff, including no less than 8 finger guns when Michael, Dwight, Andy and Pam got a little too caught up in a murder mystery game.
Real Men: Hopeless suburbanite John Ritter tries to use a finger gun to cover super agent Jim Belushi during a gun fight. Ritter comes to believe that the finger gun works when Belushi coincidentally fires at the same target. Later, an enemy is so surprised by the finger gun that he backs into a sharp stick and dies. Finally, the Big Bad is shot by a minor character who was believed to be dead, leading Belushi to believe it works.
An episode of Spaced features two epic finger gun battles, even throwing imaginary grenades and knives into the mix. The second fight has the two main characters engaging in a fight with some kids trying to mug them, and getting away while the muggers were playing dead. Finger-guns are Serious Business.
In Warehouse 13, a policeman mimics shooting himself in the head this way.
Heroes had a character who had this power, it was strong enough to make glasses shatter. He only appeared in a flashback episode, as Sylar tore his skull off, then never used his power. Presumably because it's one of the stupidest powers to ever appear in the series.
Quark pulls one on Star Trek: Deep Space Nine, "Little Green Men." As part of their attempt(s) to get out of government imprisonment, he threatens to shoot his "hostage" Nurse Garland...
Gen. Denning: With your finger?
Quark: With my death ray.
Denning: It looks like a finger to me.
Liddy, the crooked cop in the fifth season of Dexter, aims his finger at a random kid in one scene just for the hell of it.
The Adventures of Superman had Superman (as Clark Kent) jab his finger into the back of a mobster, who assumed Clark had pulled a gun on him. He actually said to the crook that it might be just his finger, and the gangster replied, "I know steel when I feel it."
In Justified when Big Bad Quarles points his left hand at you it is actually a very serious threat. He has a spring-loaded sleeve gun strapped to his left arm and when his arm is in that position he can have a real gun in his hand in the blink of an eye. When he is pointing his hand at you he is actually considering if he should shoot you for real.
In a first season episode of Cougar Town, Jules uses a finger gun on her head and uses her other hand to mimic the other side of her head getting blown off. In season two, finger gun "suicides" (and murder-suicides) become rather common. Then there's the "truth guns"...
Similar to the Xbox 360 commercial mentioned above, this forms the climactic battle in the video for "Country Song" by Seether (in a western setting with the band dressed as cowboys). After his band-mate is shot down, the lead-singer does a brief Skyward Scream before pulling out an invisible Gatling gun, mowing down everybody left before being abducted by aliens. The video takes place in the imagination of a little boy, and everybody involved are merely toys.
The David Bowie video "I'm Afraid of Americans" uses finger guns as a recurring motif, which culminates in Trent Reznor shooting a car with working pantomimed machine gun.
When the Nostalgia Critic, Linkara and Spoony teamed up for their Alone in the Dark review Spoony had to resort to this because he didn't have a signature gun like those the two others had.
Whateley Universe example: the supervillain Pointer has telekinetic powers, and prefers to blast people with a Finger Gun style. In "Ayla and the Birthday Brawl" he manages to take out nearly half of Team Kimba.
In Gym Class by Olde English Comedy, a Finger Gun fight breaks out during gym, complete with gunshot sounds, blood, and most of the cast ending up dead.
In the fourth ASDF Movie the kid mugs the mugger using his finger guns. The mugger lampshades that it doesn't make any sense.
In an episode of The Goon Show, two army officers discuss a recent incident with the show's moronic protagonist Neddie Seagoon:
"During that last German attack, all he did was point his finger at them and shout, 'bang, you're dead'."
"Perhaps he had run out of ammunition?"
"No! I inspected his finger, and it was fully loaded!"
It's been a signature mannerism of his throughout the whole series. "You're pretty good, kid."
In Metal Gear Online, players who draw Liquid Ocelot have access to his unique Guns of the Patriots weapon, which is a Finger Gun. He points it at enemies and says "bang", and locks their weapons for a few seconds.
In Team Fortress 2, the Heavy Weapons Guy has this as one of his taunts, and it can insta-kill anyone unlucky enough to be in his line of fire.
Reisen from Touhou was shown doing this (kinda...) on official art, and thus it became her personal gesture in Fanon. She also actually shoots bullets this way on the Fighting Game Spin-offs. Of course, they're not actually bullets, it's magical danmaku.
In Dawson's Cinekill in Eternal Champions: Challenge From the Dark Side, the Dark Champion challenges him to a showdown. The Dark Champion's weapon is a finger gun... which he loads with the bullet from Dawson's gun. He turns out to be faster on the draw.
Several in Borderlands, namely the psycho on the cover of the game, and bandit lords Krom and Flynt.
Goofy finger-guns himself in the head after missing an easy putt in the Classic Disney Short "How to Play Golf." Yes, it goes off. And yes, he only gets singed.
When the boys from South Park were playing police, they used Finger Guns. When they were deputized by the real police and sent in to bust a meth lab, and later to take down the leader of a drug ring, they used Finger Guns as well.
This was what they thought the phrase "Finger Bang" meant, and thus what they named the Boy Band they formed.
The season one finale of Total Drama Island has a more realistic use of this. While being forced to listen to Harold's long and boring speech about beavers Noah pretends to shoot himself with a Finger Gun, even making a bullet sound to go with it.
In Johnny Bravo during the episode the titular character partakes in a marathon, the guy with the start gun uses this trope instead, pointing in the air and shouting "BANG!" and results in smoke rising from his finger tip. He stares confusedly at his finger while a dead bird falls from the sky.
Also in the pilot episode, the mugger sticks up the gorilla with a finger gun.
An episode of Futurama features the characters playing a virtual game of laser tag, in which the Finger Gun is the primary weapon. Fry at one point even uses a "Fist Shotgun", "cocking" his wrist with his other hand and unleashing a laser blast.
In the Tex Avery-directed cartoon "The First Bad Man", when his real gun runs out of bullets, Dinosaur Dan uses his own finger as a gun instead.