For every given male character, there is a female version of that character, and vice versa.
They may be canonical. Most of them aren't. The internet has a fascination for producing gender swapping regardless of the context of the original character. Canon-derived examples of the trope tend to outlast their actual-canon inspirations (which are often single stories, or even short throwaway gags.)
No character is immune to this effect — not Darth Vader, not Master Chief - no one. note Not even Chuck Norris. Rule 61 lied to you.
Closely related to Gender Flip. When a separate character is a female knock-off of an existing male character, you have a Distaff Counterpart. Crossdressers, of course, provide their own version of this. There's a good chance that you'll see the character in question in a Gendered Outfit (like our lovely Maria at right, here).
One of the Rules of the Internet. Can (and often does) overlap with Rule 34. Since these female versions are often much cuter than their male counterparts, it can also overlap with Self Fanservice.
Haku, who appeared early in the Manga (and again much later). A large portion of the community (especially Fan Fic writers) seems to like the idea of Haku actually being a girl, either omitting the one actual indication of his gender or just simply having Haku and Zabuza lying about his gender.
In the first season, Ash Ketchum dressed up as 'Ashley', a little blonde girl in an orange dress. She's still drawn by the fans, has her own fan clubs, and some people consider her a different character.
Ash has been forced to crossdress once each region after this—as a one-shot female NPC in Hoenn, and as a French Maid in Sinnoh. Guess what else has lots and lots of fanart.
Ash/Ashley. Yes, it exists. Wouldn't be surprising if there was also Ashley/Ashachu or Ashley/Ash/Ashachu *
Black Lagoon does the gender inverting in one of its specials seen here on youtube, including an awesome male Balalaika (and Eda looking an awful lot like the cowboy mercenary she had a Gun Kata duel with). Hilarity bonus also runs for the Creepy Twins. The best part is that, in spite of having their genders swapped, the characters still have the same voice actors, who make no attempt to sound more gender appropriate.
A recent chapter of the Soul Eater manga made this canon. Most of the main characters (including Blair, but sadly, not Kid) became gender-swapped. How long a character stayed in his/her gender-bended form depended on how much of a Covert Pervert that character is.
Through the magic of pitch-adjusting technology, many of the K-On! songs have been gender-flipped. As have the main four characters. Azusa doesn't seem to be gender-flipped yet since they seem to have been made before she was introduced. Here's the Season 1 opening (Cagayake! Boys), and the rest can be found here. The real kicker? It's actually pretty darn good.
The title character of Ranma ˝ is an canonical Gender Bender, being cursed to change into a sexy, female version of himself on contact with cold water, because he was once tossed into a cursed spring in a training session by his idiot father. While obviously a "real" aspect of the Ranma character, fandom has taken the concept even farther, naming the female side of him "Ranko" and making her an actual character in countless fan works, (many of them extremely NSFW).
According to the backstory of Fate/Stay Night, Saber is a gender-flipped version of King Arthur, who pretended to be male during her life, with some help from Merlin.
Main characters of Durarara!! also aren't safe from this. Here's the proof◊
Hayate of Hayate the Combat Butler was found to play a very convincing female (even attracting the romantic attention of three male characters in that form). His female appearance has since made several more appearances and its own name (Hermione Ayasaki) and has been fandom-paired with his gender bent form.
Ren/Run from To Love-Ru can somehow change genders by sneezing. Lala also invents a device that temporarily causes someone to switch genders.
Parodied in the anime version of Cromartie High School. The first half of the last episode is devoted to reusing jokes from the beginning of the series, but with the central cast's female versions, who have the exact same voices and unattractive faces as their male counterparts. (Fanart of these versions is very rare.)
DC's AnimesqueAme Comi Girls line has female versions of many male characters, but not vice versa, and not for all male characters. Most are actually the DistaffCounterparts of the male characters, "promoted" in their absence. But there are a few straight examples of this trope, including a female Sinestro, and an Indigo made indistinguishable from a female Brainiac.
Marvel Mangaverse: Characters who don't start the story off already female are replaced by females very quickly.
Genderbending Loki from The Mighty Thor isn't that difficult to imagine since not only is he a shapeshifter normally, but he assumed Sif's form for several story arcs for the lulz. Princesses Thor and Loki aside, The Warriors Three have also enjoyed time as the fairer sex.
An issue of Lobo has the Main Man dying and going to the afterlife... only to be reincarnated as an extremely masculine female version of himself.
Dennis the Menace (UK) had a female cousin who looked was exactly like him but wore a bow in her hair and a skirt. Also her name was Denise The Menace.
A canonical Superboy story, "Claire Kent, Alias Super-Sister"note Superboy #78 (1960) has Superboy transformed into a girl. It's not permanent ( and, as it turns out, the whole experience was just a mental illusion), and the story is mostly forgotten today, except as a tryout of sorts for the Supergirl concept. Mostly. There is quite a lot of fan art, and several Fan Fics, based on the premise that Clark was permanently stuck as Claire.
Pops up every now and then on Sliders, Most notably in the episode Double Cross where there is a female counterpart of Quinn named Logan St.Clair.
Star Trek: The Original Series, in its last-aired episode, "Turnabout Intruder", tells the story of a forced Body Swap between Captain Kirk and his crazy ex-girlfriend Janice Lester. Of course, everything is sorted out at the end... though that hasn't prevented a plethora of Fan Fics, fan art, and photo manipulation about alternate scenarios where Kirk is stuck permanently in Janice's body (usually due to Janice-in-Kirk's-body being killed.)
Female Space Marines◊ pop up now and again, but are often met with cries of Canon Defilement (also known as HERESY!) due to a supplement which states that Space Marines are explicitly all-male.
Yu Gi Oh: Here's Fabled Dianaira Its name might come from 'Deianira' in the Greek mythology. Ironically, Deianira is a female while this card is obviously male. However, a fanartist "corrected" this mythological error.
Partly thanks to the entire cast being male, a significant amount of Team Fortress 2 fanart/fanfiction is Rule 63. The most frequent target is The Chick, Medic. As stated below, Nerf NOW!! has them as characters.
ChemicalAlia has a mod to replace the Scout, Heavy, Engineer, Medic, and Spy (with more coming) with female models and fan-made female voices.
From the same maker, BlazBlue experiences the same thing with female versions of Ragna, among others.
Lost Saga not only loves to include BlazBlue characters into its game, but also allows you to buy an item that lets you play as a female version of that character, examples include Ragna and Hazama, Jin is the only exception as his female version is Noel Vermillion.
It's become surprisingly popular to depict MOTHER's big bad, Giegue, as a female. Since Giegue's a skinny grey alien, some non-canonical anatomy comes into play.
He is not the only one that suffers (or is very popular) due to this rule. In fact most of the King of Fighters have been subject to this at some point or another (save Mai for some reason). But the one that gets it the most (even more than Iori) is Leopold Goenitz of the Wildly-Blowing Wind. Yes, thatguy... Oh and yes, Demitri'sMidnightBliss is the one to blame for Goeniko. There are other characters based on this concept that simply run with the idea, and the Mugen fanbase tend to make a character out of this.
Shovel Knight plans to handle this in a unique way by creating an entire MODE for it. "Gender Swap Mode" will flip the gender of every main character, from the protagonist, to the bosses, to the Big Bad herself.
Nerf NOW!! has produced female versions of all the Team Fortress 2 characters, as well as several characters from other games, such as Starcraft. And they're all adorable.
Seeing as anyone and everything gets shipped together in the Homestuck fandom, this was inevitable. Gender-bent versions of Dave, John, and the Midnight Crew are particularly popular though. Of course, genderbent versions of John and Jade are quite hard to distinguish from the original, since they're so alike they could as well be Half-Identical Twins... oh wait, they are.
There are tons of examples on Bittersweet Candy Bowl. It started off as a favorite thing of the fans to do, but then spread to the author. Most of the time it leads to the most insane Crack Pairings.
Although Garii in Ménage ŕ 3 is just a figment of Yuki's imagination, she's close enough.
Scandinavia and the World has a male and female version of most commonly mentioned countries, they are generally very similar in appearance (Sister Germany even has her brother's mustache).
Many characters in Arthur, King of Time and Space have different genders based on their arc. Tristram, in particular, is female in every arc besides the baseline, and Morgan and Nimue are actually male in the M*A*S*H* arc.
An official version of this occurs in Ears for Elves. The last panel here calls the rule by name and shows what Tanna and Luero would look like when gender-flipped.
Tumblr users really like making female versions of Gabe and Tycho (named "Gabbie" and "Tyche", respectively). Zeboyd even responded to it by adding a patch to On the Rain-Slick Precipice of Darkness Episode 3 that allows players to play as the gender-flipped Gabe and Tycho if they want.
Jix has a story arc that revolves around a device that might as well be called "The Rule 63 gun." Lauren spent the majority of the arc as a feminine looking man.
At one point, there was also an amusing series of videos created by (female) TGWTG fans dressing up as their favorite (male) reviewers and demanding that the fandom fix this crackfic singularity gender bend... thing.
Played with in Red vs. Blue: In addition to the six inept, clueless, lazy, and outright retarded men, who because of the series being a Halo machinima all look identical with only different color, there is a single female character in the form of Tex, who is almost completely sane in comparison and a more capable soldier than all the others combined.
Happy Tree Friends plays this differently depending on which side of the fandom you are looking at. Among Japanese fans, the characters who get genderbended the most often are the four originally female characters. Out of the boys, designing female Russell, Lumpy and possibly Flippy seems to be the most common, although it is easier to tell who never is bent. Most of the results from searching for genderbends by the western fandom are either Flippy as a woman or female copycat sues based on him.
Metalocalypse. Dethklok had a tribute band that was composed of all women, giving each of the band members a female counterpart. Dethklok rejected them, as they found them rather creepy. Though Toki admits the female Pickles was cute, and Nathan agreed she did have nice eyes.
One particularly nice example of the band getting genderbent exists here.
Superjail has an episode where female versions of the entire cast appear, complete with a female-to-male transgender to compliment Alice and a lesbian couple in the place of the gay couple in SJ.
If there's ever going to be a fic or discussion about the gender-bent characters, there's a host of accepted names. Twilight Sparkle is Dusk Shine, Rarity is Elusive, Fluttershy is Butterscotch, Rainbow Dash is Rainbow Blitz, Pinkie Pie is Bubble Berry, Spike is Spines (or Barb/Barbara), Celestia is Prince Solaris and Luna is Prince Artemis. Only Applejack remains the same.
This was played with in "Magic Duel" - Twilight Sparkle transforms a mare - Applejack - into a stallion for a short time. It's later revealed that it was actually her brother painted in Applejack's mane and coat colors.
"Neutopia", the seventh season premiere for Futurama, features Gender Bender versions of each character. The episode even lampshades this trope when a calendar of the sex-changed characters makes a huge profit due to the "creepy fans" of Planet Express.
Osmosis Jones fan art examples have been seen, including cosplay of a female Thrax.
In an early episode of Young Justice, Miss Martian uses her shape-shifting powers to imitate Kid Flash, Robin, and Aqualad — but female versions, because her powers aren't developed enough to readily do more complex shape-shifting stunts such as mimicking the opposite sex. Kid Flash approves.
Every Vocaloid (including Rin and Len) and almost every UTAUloid tend to have a genderbend. In the UTAUloid's cases, many are made by the authors themselves. People have also edited their vocal files to create a masculine/feminine version of the characters.
Among Mixed Martial Arts fandom, Cristiane "Cris Cyborg" Santos is considered to be Wanderlei Silva's female counterpart.