A fashionable bit of personal accessorizing are Femme Fatalons. Often filed to a painful point and extending well past the end of the fingertip, these nails mark the woman wearing them as, at the very least, untrustworthy, and often as dangerous or predatory.
It's uncanny how often women with Femme Fatalons turn out to be villainesses too. While Power Hair at least has the benefit of the doubt (mostly because most people have hair), it's a rare woman who can wear these and be on the side of good. Maybe it's the claw-like look, the illusion of Creepy Long Fingers, the implied sadism and Finger Licking Evil, or perhaps just having them is itself a Red Right Hand (well, red fingernail polish).
Non-natural long nails, like acrylic or gel extension or glue-ons and primarily decorative nail guards or nail sheathes (these) also count as Femme Fatalons, as long as they're used as a hint that the wearer is dangerous or villainous.
Weapons like clawed gloves, or clawlike extensions attached to (or growing out of the hand or fingers) are not Femme Fatalons; they're Wolverine Claws.
Despite the name, Femme Fatalons are not only worn by women, either. The occasional male villain will opt for them as well.
Compare Finger-Lickin' Evil or actual claws. Often found along with Fangs Are Evil. May be paired with I Broke a Nail.
In the first opening of Toradora!, there's a close-up shot of Ami Kawashima blowing a bit on her manicured fingernails.
The latest episode of Kiddy GiRL-AND introduces Rubis, an assassin who transforms into a animal-like creature and extend the nails on both her hands and feet as weapons. Also an example of Finger-Lickin' Evil.
More male examples: Naruto Uzumaki when he uses the Kyuubi powers, Sasuke in his cursed state, and occasionally Kiba Inazuka.
But Nii Yugito, the Nibi jinchuuriki, who is a woman, has the longest nails by fair, on both her hands and feet.
Lilly Five team member Ellie in Shaman King has a french manicure which serves as the medium for her oversoul which becomes a set of claws.
A woman in the second Detective Conan (Case Closed in America) movie has press-on nails that glow in the dark. She is killed as one comes off as she struggles with the killer, and the characters observe this and what press-on nails are to begin with.
In the first episode of One Piece, the evil Alvida has long, sharp red nails.
A number of characters from Dragon Ball have this trait, perhaps most noticeably Namekians because the first one of them introduced was imagined as a demon. Unlike most of the examples on this page, they're a male looking One-Gender Race.
Lust from Fullmetal Alchemist has the power to grow her nails incredibly long. They can also retract into her hands and must be pretty strong, since they never break. Although she does use them for slashing, mostly, she uses them to stab people.
The cover of Sabrina the Teenage Witch #90 uses this as a metaphor for Sabrina being tempted to The Dark Side—one hand has rounded nails with pink polish, while the other has sharp nails with a coppery polish.
The Shade Durza in the film version of Eragon has these.
John Malkovich, I mean King Galby, also has press-on nails.
Casanova Frankenstein, the villain from the film Mystery Men has these. Used for comic effect, as he prefers to attack with the sharpened nail of a single outstretched pinky.
Jada Pinkett Smith's character Peaches in A Low Down Dirty Shame.
The Brazilian horror character Coffin Joe has ridiculously long fingernails, which he uses to gouge people's eyes out.
In the French action movie The Nest (Nid de guêpes), villain Abedin Nexhep is introduced with a shot of his long (for a man) and manicured nails. Any notions you might form about him being a cossetted hands-off type is later gruesomely punctured, along with the throat of one of the cops.
In Dennis the Menace, Dennis' mother's antagonistic boss has red fake nails and even taps them on a divider as she walks along in one scene.
Winifred Sanderson from Disney's Halloween movie Hocus Pocus has these. With all her crazy hand motions, it's kind of hard to miss, especially when she uses one like a knife to cut through a window screen.
Bellatrix Lestrange had long nails in several of the Harry Potter films
In Skyfall, icy henchwoman Severine wears these, laquered purple and gold. They go with her absurdly vampy beaded dress, and were intended to evoke the appearance of a dragon.
One of the female smugglers in Flood Tide has long fingernails.
The Warden of Holes paints her nails with polish made from rattlesnake venom (and then scratches her minion with them when they are still wet, causing him severe pain and injury).
Another male example: the Star Wars EU character General Melvar has very long fingernails. Very long, metallic, reflective, sharp fingernails. He's even shown to have cut himself with them by accident before. Aaron Allston's run on the X-Wing books, basically a canon Fix Fic for the book in which Zsinj and Melvar were introduced, explains that this is part of an act to make himself look like a stereotypical evil/psychopathic Imperial, both to make his enemies underestimate him and to impress/intimidate pirates he's trying to recruit.
The Chancellors of the Capellan Confederation in Battletech wear the last three nails on each hand extra long and extra sharp following the fashion of Chinese emperors in Real Life history. Of course, the emperors weren't half as likely to have them artificially reinforced...
Coraline: The Other Mother has long thin fingers with red nail polish on each fingernail.
There's never any doubt that Drusilla from Buffy the Vampire Slayer is evil, but she definitely has these. She kills Kendra, a Slayer, in the season 2 finale by cutting her throat with a single nail.
A Yellow Peril villain from Get Smart had these, going as far as to cut up apples and carve statues with them. Agent 86 comments that this is probably how he is able to afford an underground lair. "Think how much money he saves on knives!"
Numerous villains in Charmed (both male and female) have had long nails.
In Farscape, Aeryn Sun's mother apparently had these, as she escaped capture by using them to slice open her own arm to pull out a knife she apparently had surgically implanted just in case.
In "Incubator," Scorpius' nanny had red, talon-like nails. For that matter, so did Scorpius as a child, though he evidently trimmed them before he started wearing his coolant suit.
One minor character had long nails complete with a diamond piercing in the pilot episode of Dark Angel. She even went so far as to offer to show Max how sharp they really were.
In the episode "Prom Night" of Full House, one of the main female characters wears very long, red fake nails for the prom, and they get some attention. In the end, they mysterious come off without her noticing and get stuck to her date's rear end.
In an episode of Oz, Broken Cutie Tobias Beecher lets his nails grow long and sharpens them so he can slash one of the prison guards to death and then destroy the murder weapon simply by clipping his nails.
The Rider in Bally's Centaur shows off some sharp fingernails on the playfield.
Role Playing Games
Most cyberpunk-style games have some variant of these. In Cyberpunk 2020, for example, they have "Scratchers", which are implanted metal or carbo-glass fingernails, though they're designed to cut on the bias, so you have to slice crossways, not rip downwards.
Shadowrun also has them, in the form of "Hand razors", which are 2.5-centimeter chromed steel or carbon fiber blades that either replace the user's fingernails or slide out from beneath synthetic nail replacements.
Rifts has "Razor Fingernails" which can be used in a slashing or clawing attack.
Even BattleTech has them. One of the affectations of the wealthiest citizens of the Capellan Confederation is to grow the fingernails on the last three fingers of each hand to a length of ten centimeters and decorate them with gem chips and gold leaf, and reinforce them with carbon fiber, turning them into claws able to cut through leather.
A lot of actors who play Herbert von Krolock in Tanz Der Vampire have these. Some just have the nails attached to gloves, however, and sometimes, his hands just look exquisitely manicured.
In The Golden Apple, Achilles warns that Circe will "scratch your heart out with her long red fingernails."
In The Women, Sylvia in the first scene is showing off her nails, which have been painted a new shade called "Jungle Red." Nancy says it's good "for tearing your friends apart," a quip which Sylvia resents. Mary admires the shade, but the chatty manicurist who gave it to Sylvia also divulged some gossip about Mary's husband keeping another woman. At the end of the play, Sylvia and the other woman are sore at Mary for having cattily outwitted them, and she says, "Well, I've two years to sharpen my claws," and flashes her fingernails—"Jungle Red, Sylvia!"—before leaving the two ladies alone.
Many of the female villains in the Lunar series had long, manicured nails.
Elexis Sinclair from the PC game Sin: Episode 1 has long red nails.
The Transformers original series had the (male) Decepticon Scourge with bright pink sharp "fingernails".
In Transformers Prime, the evil characters have much sharper fingers than the heroes. Starscream's claws actually seem to vary in length depending on how evil he's being in that scene.
Azula and Mai from Avatar: The Last Airbender, though they later cut them. Although Ursa (nice lady, from what we can tell) had them too, so maybe it's just Fire Nation fashion for nobles and royalty. They also appear on Hamma, as both her fingernails and as water-bending produced claws which she throws at a tree.
Azula also uses the slashing motion in combat in her first episode, giving Zuko three nice straight scars.
The first hint that Hama isn't some sweet old lady is her using water she sucked from plants (killing them) to create talons of ice.