A Hentai Manga take on this: WereSlut (unfortunate name created from whole cloth by Eros Comix, the US distributor) is about a shy college student named Kimiko who takes a magic beauty pill to make her attractive enough to win her childhood friend (read: she takes off her glasses). It works, but, due to the magic pill being past its expiration date, it keeps kicking in whenever she gets aroused. After the first time, though, it also kills all her inhibitions.
Ditto for the manga/anime Sprite: Between Two Worlds,wherein Manami, a severely repressed and meek college student turns into Nami, an outrageously uninhibited flirt whenever she's put into any situation that is too much for her to handle. The catalyst is her cousin Toru, who convinced Manami to be more confident around him when they were young by convincing her she was really Nami.
In the ecchi manga My Balls, the main character's primary love interest Minayo is a sweet, kindly young woman who works at a daycare center. When she gets drunk, however, she goes into what the protagonist describes as "full-blown nympho mode". Unfortunately, said protagonist can't have sex for a month or the demon sealed inside his balls (hence the name of the manga) will be released and cause The End of the World As We Know It. Minayo happens to be an alcoholic, and Hilarity Ensues.
Paranoia Agent has an episode that plays with this. Harumi Chono is a quiet, conservative, mild-mannered school teacher by day, but at night her other personality takes over—Maria, a popular prostitute. When Harumi gets engaged, she starts trying to desperately get rid of Maria before her wedding day, which causes Maria to fight back.
Code Geass's NinaEinstein and the infamous table scene. Nunnally actually walked in on this, but fortunately she was blind already and (hopefully) too young to properly...interpret any sounds she might have heard.
Paprika: Straight-laced Chiba becomes a lot more flirty and extroverted when she turns into Paprika.
In a way Nyamo-sensei of Azumanga Daioh. While she's generally straight-laced and down-to-earth in contrast to Yukari, the one who you'd expect to have a wild social life, Yukari makes a comment about her Nyamo "not being alone" in a previous summer, and Relaxo Vision or not, the hilarious incident where she gets drunk and gives The Talk implies she's fairly experienced.
In the manga Crimson Spell the protagonist is under a strange curse that causes him to turn into a Demon that can only be vanquished by being raped into submission. Conveniently he never has any memory of this the next day.
While it never came up in the comics, writer Mark Waid's character notes for the Fantastic Four speculate Susan Storm was like this as a young woman to cope with the stress of having to raise her little brother.
Aurora was like this in Alpha Flight's early years, exaggerated by her Split Personality. One persona was Jeannie-Marie: strict, stuffy schoolmarm teaching at the same girls' school she was raised in. And then there was Aurora: sultry, adventure-loving, free-spirited partier.
In A Pikachu in Love, Pikachu is worried that this is what Ash and Co. will think if they catch him sneaking off at night to meet Pichi, and doesn't want them drilling him for questions on the matter. That still doesn't stop Misty from finding out, though she doesn't press the matter and keeps it a secret from the others even after the ending.
A Pleasant Surprise paints her as being quite assertive and forthcoming with her sexuality to the point of outright seducing the protagonist.
Blissful Dream is another such story. Here, Fluttershy is shown as being sexually dominant over her lover.
Advice From Pinkie Pie puts Fluttershy in a BDSM relationship - as the dominant one - with Rainbow Dash ("Now, my little pony… you're going to love me."), while in My Little Slave, she initiates a rape-dungeon roleplay that lasts for 29 chapters before the reader finds out the truth.
The movie Belle de Jour is about a seemingly average housewife who's a prostitute in secret. But here this trope is inverted: as she is a housewife, she can actually do this job only by day instead of by night. (This is where the name of the film comes from.)
Somewhat subverted in the film Blind Date where Bruce Willis' sister-in-law tells him not to let the date get drunk as she goes wild. This information is relayed to him via his brother who adds lots of leering to the information. Cue the man getting his date (Kim Basinger) drunk, to find that she goes wild in a drunk-driving, crash-the-car, ruin-the-evening way.
Looking For Mr Goodbar is a very dark example. Diane Keaton plays a teacher of deaf children from a conservative Irish-Catholic upbringing who starts out having an affair with a married college professor, then starts cruising seedy bars for one-night stands that grow progressively more degrading and dangerous, culminating in her getting raped, beaten, and stabbed to death by a troubled Vietnam vet.
The Torchwood novel SkyPoint takes this to extremes; a very prim and proper little old lady, who is actually an S&M prostitute.
Almost every Cairheinan noblewoman in the Wheel of Time is heavily implied to fit this trope. Though during some festivals they will run around almost nude on the streets, especially during the Feast of Light (during which normal prohibitions against inter-class kissing (and other things) are lifted), leading to a large number of commoners having bragging contests about who made out with the most important noblewoman.
ONLY during the Feast of Lights, after which any such incident is ignored by all parties. With Cairhien being partially based on Versailles France, this roughly corresponds to Mardi Gras.
A relatively tame version occurs in Good Omens. Sweet little old lady and part-time medium Madame Tracy also offers appointments for "strict discipline and intimate massage every evening except Thursdays", and there are hints that she goes a little further than that on Thurdays.
In Saving Zoë, Teresa is a pretty preppy girl who acts and sings. By night, she hangs out with drug dealers and porno producers. The same could be said about Zoe, who seems like a perfect bubbly girl but smokes drugs and has sex when her parents don't know. This tendency leads to Zoe's death.
Live Action TV
On The Buses long-suffering Olive would love to be Naughty By Night, but her miserable husband Arthur would rather sleep. Actually, he'd probably rather do his taxes...
Twin Peaks - Laura Palmer is a normal high school girl by day, but a prostitute at night, among other things. Or, as she says in The Movie, "Night time is my time."
Reno 911. Deputy Weigel and her boyfriend Craig both appear to be socially-awkward nerds, but what we see of their sex life is quite... adventurous. Subverted in that Craig turns out to be a serial killer.
Subverted on Person of Interest where a PoI is a dedicated workaholic doctor during the night but seems to spend her nights going to night clubs and picking up men. However, a more detailed investigation reveals that she is actually stalking the man who raped her sister and caused the sister to commit suicide. The doctor is simply building up her resolve to murder the man.
the Good Charlotte song Like it's her Birthday has this as the main theme - the Protagonist thinks his Girlfriend's cheating on him when she lies about where she's going, so he follows her and...
The play Miss Julie fits this with regard to personality, but replace "librarian" with "aristocrat".
Fantine in Les Misérables is fired from her factory job when she is discovered to have a child, and her fellow workers claim that she is a prostitute. However, she had only ever had one lover, who had abandoned her, and it is only after she is fired that she has to become a prostitute.
The song "Miss Byrd" from Closer Than Ever is about a secretary exulting in how no one who sees her would think that she's anything but "that dull Miss Byrd," yet she has secret trysts with her superintendent.
Parodied in The Music Man: The town gossips manage to convince the Professor that Marian the Librarian is Naughty by Night primarily because she reads "racy" books by "Chaucer...Rabelais...Balzac" and was kind enough to befriend the town outcast. She's not, but he falls in love with her anyway.
Question 18: Honor student by day... stripper by night!
Possibly Litchi from BlazBlue. Despite her incredibly skimpy battle clothes, her personality is fairly conservative. However, supplement materials and a couple of gag endings say that when she gets drunk she gets rather...insistent.
In Futurama, a prim bureaucrat is utterly turned on after hours by Fry's horrifyingly slobby lifestyle.