Suzumiya Haruhi no Seitenkan, or (roughly) "The Gender-Flip of Haruhi Suzumiya" in English, is a fan creation based on the Suzumiya Haruhi franchise that serves to tell what the universe would be like if all the male characters were female and vice-versa. Started off as just a random musing that became quite popular because of the difference in dynamics from the flipped gender roles and dynamics that gives a distinctly different impression from the original. Such as changing Kyon's cool snarks into Kyonko's cute sass.It got so huge that it is one of the longest threads in the Animesuki forum and gave birth to the revision projects. Also notable for having strikingly different versions of genders with Kyon(ko) [you really can't tell that they are genderflips of each other on appearance alone], that they might as well be two unrelated characters if not for the personality and origin.
Tropes featured in the gender-flipped fanfics include:*
Note that not everyone in the fandom uses the same gender-bent names; the main variation is "Haruhiko" for Haruhi, which is pretty common in the Japanese fandom.
Crossdresser — Poor Mitsuru, forced to wear a frilly maid dress for a photo shoot and whenever he shows up at the club room for the remainder of the gender-flip of the first light novel!
Double Standard — Why some people find Haruki considerably less bearable than Haruhi even when they are acting the same way. Also, arguably the whole point of the Rewrite Project, changing the gender of all characters, placing then in the same situation and seeing how things would change.
Guilty Pleasure — To Kyonko, the photos of Mitsuru in the maid dress and herself in the bunny girl costume, to the point where she saves the photos to an encrypted folder instead of just outright deleting them.
Fanship couple Tsuruya(M) and Kunikida(F) tend to be portrayed◊ as an odd couple like this. Somewhat better justified than other interpretations since in the original Kunikida was a shorter than average guy and Tsuruya was a taller than average girl.
Actually, everyone get the at last first set of the image songs genderbended. Most are just pitch altered versions, however, so they might sound a bit weird (although the voice sounds similar). Some, like the 3 above links, are actual Fan Dubs.
The Password Is Always Swordfish — Unfortunately for Kyonko, Haruki was able to access her hidden "MITSURU" folder by using her brother's name. And people wonder why it's common courtesy to not put personal information on your computer, and not just in that way—Haruki, upon seeing the Playboy Bunny photos of Kyonko, considers forcing Kyonko to wear said costume at all times when she shows up to the club room.
Pink Girl, Blue Boy: Very subtly. In the rewrites, alongside all the gender flips, the celestials in closed space and the glowing espers had their colors switched; the blue celestials were made red, and the previously red espers were made to glow blue.
Playboy Bunny — Kyonko was unlucky enough to get a bunny girl costume as a present from Haruki, who expects her to be grateful and try it on.
Promoted to Love Interest — A variant of this trope in that female!Kunikida and male!Tsuruya are shown to be very interested in each other in several fan works, whereas in the original their interaction was much more limited and nonromantic. At least until the 11th book, where they got some Ship Tease. Ascended Meme, perhaps?
Sacred First Kiss — Kyonko, just like Kyon, has to give hers up to Haruki to stop him from destroying/rewriting the world. It's a bit more... intense... than it was in the original novels. And the description was a bit more thorough.
Spell My Name with an S — Pretty much everyone, as the Gender Flipped Haruhi project began in a disorganized fashion:
Suzumiya - Haruki is how he is known by the English Fandom, however, Haruhiko*
As Haru-hiko, not Haruhi-ko
is more common in Japan.
Nagato - Usually Yuuki, although Yukio is not unheard of.
Koizumi - Japanese Fandom calls her Itsuki, with her name spelt using more feminine kanji. English speakers have to differentiate somehow, so we call her Itsuhime (hime being an alternate reading of the kanji in Itsuki's name) or, as is more common now, Itsuko.
Asakura - The "correct" spelling is Ryou, but is commonly spelled without (Ryo).
Kimidori - Emori? Emirio? Eriotto? Elliot? No one knows for sure.
The point of "Emirio" and "Eriotto" was picking a similar sounding western name and write it in a Japanese way. It should be noted that "Emiri" is an actual Japanese name and not (in this case) a corruption of "Emily".
Kyonko - The only character who don't have variations on it (nick)name. Some still prefer to call her 'Kyon', tough, as Kyon is a nickname, so it wasn't deemed necessary to change it. Anyway, when there's discussion about Kyonko, she's usually referred to with the "ko" for easier identification.
Stripperiffic — Haruki and Mitsuru (the latter unwillingly) dress up as Chippendale Dancers while passing out flyers advertising the SOS Brigade.
Visual Novel — The Crossdroads of Haruhi Suzumiya, a yet to be finished fan-made Visual Novel that switches back and forth between the normal and gendenbent universe. That is, until Kyonko becomes a Slider and accidentally brings everyone to the original universe.
Would Hit a Girl: Haruki gave a good kick at the Computer Club President's head.
Zettai Ryouiki — Itsuko, the Computer Club President, and the Student Council President. Also suggested for Kyonko at one point.