Mona Lisa Smile

She's smiling because she knows she's spending her time more productively than you are.

"...in this painting of Leonardo's there was a smile so pleasing that it seemed divine rather than human; and those who saw it were amazed to find that it was as alive as the original"

—Giorgio Vasari, Lives of The Artists (trans. George Bull)

Because the Mona Lisa is such a famous piece of art (Small Reference Pools at work again), the painting in its entirety—or the lady-in-the-painting's enigmatic, not-quite-reaching-the-eyes coy smirk—show up quite a bit in Homages, Shout Outs, and outright parodies.

Has nothing to do with the film of the same name.

Examples of Mona Lisa Smile include:

Anime

Toradora! - Taiga, after torturing Ami by forcing her to do multiple impersonations, uses threats of impersonations to come in order to control her. The best of the bunch? "Mona Lisa taking a corner at 250 miles per hour." Just picture it for a minute and TRY not to laugh.
Yoshinoya-sensei from Hidamari Sketch has the painting.

Comic Books

The Batman Elseworld "Dark Masterpiece" parallels a modern art-theft (by the gallery owner) being prevented by Batman with the kidnapping of Lisa del Giocondo herself (by her husband) being prevented by Leonardo's apprentice in a Renaissance-style bat-costume. It ends with Leonardo asking Lisa what her secretive smile is about, and being told that she's pregnant, and he's the father.
In Booster Gold, the Mona Lisa is actually Booster's sister (at least partly, Michelle is blonde, so presumably he found another model to finish it after Booster dragged her away).
- Actually, Booster comes back to get his sister and tells Leonardo he's taking her back to Rome. Leonardo, who had the face all done, struggled to grasp the rest of her after she left. He decided, due to personal preference, to give her dark hair and change her clothing because she had been wearing her costume while he was drawing...and that would have looked stupid.

Film

At the beginning of Hudson Hawk Leonardo da Vinci is shown with the Mona Lisa, which is finished except for the mouth. Then we see why: when the model opens her mouth, she has very bad teeth. The joke is that Leonardo decided to give her the enigmatic smile so he wouldn't have to show the teeth.
- This joke is also used on Time Squad.
Mona Lisa is one of the paintings captured and burned in Equilibrium
Parodied outright in Asterix & Obelix & Cleopatra. To their credit, Monica Bellucci makes a hot Mona Lisa.
In the Spanish short film La Gioconda Esta Triste (The Mona Lisa is Sad) the Mona Lisa stops smiling, and so does every single painted, drawn, photographed or videotaped reproduction of the painting. Not only that, but at the same time everyone on Earth also forgets how to smile. Against a background of international tension, environmental destruction and terrorist threats scientists try to solve the mystery. They finally conclude that the Mona Lisa's sadness represents that of the entire world, and only way to save the world is for everybody to try to smile again. But it's too late - everybody starts weeping, and the world ends.
Ever After - Leonardo shows off his latest work (the famous painting, inaccurately depicted as on a medium that can be rolled and stored in a tube) when introducing himself.
The film Mona Lisa Smile. Obviously.

Literature

Parodied in The Far Side, with a painting of some guy with a gigantic nose and one of the viewers comments "Gad that's eerie, wherever you go the nose follows you!"
Discworld's Mona Ogg, painted by Leonard of Quirm, and implied to be of the witch Nanny Ogg in her younger days. The teeth follow you around the room.
- This is the cover of The Art of Discworld.
Crowley in Good Omens posesses an early sketch of the Mona Lisa, which he thinks is superior. Apparently, Leonardo agreed:

"I got her bloody smile right in the roughs," he told Crowley, sipping cold wine in the lunchtime sun, "but it went all over the place when I painted it. Her husband had a few things to say about it when I delivered it, but, like I tell him, Signor del Giocondo, apart from you, who's going to see it? Anyway ... explain this helicopter thing again, would you?"

There's a passage in The Godfather where Lucy Mancini, after Sonny Corleone's death, tells her new lover about her affair with Sonny. When she says, "I did everything with Sonny," she's got what the new guy thinks of as a Mona Lisa Smile, and he can't help wondering if this explains what La Gioconda was smiling about.

Live Action Television

Used a few times in Terry Gilliam's animations for Monty Python's Flying Circus.
In the Doctor Who serial "The City of Death" the villain gets Leonardo to paint several copies of the Mona Lisa so he (the villain) could sell them multiple times to facilitate his evil plot. The Doctor foils this plan by writing "This Is A Fake" in felt tip on the boards Leonardo uses. Naturally, this means the one hanging in the Louvre also has "This Is A Fake" written under the painting, but the Doctor says that if people have to x-ray it to know if it's any good, they deserve to think it's a fake.
Mona is taken out of her portrait so Alex can cheat on a test in Wizards of Waverly Place. She ends up going back in wearing one of Harper's home made necklaces.
She also comes out of her portrait in The Sarah Jane Adventures. She traps people in paintings and searches for her "brother", another painting made using the same oils.

Music

The Nat King Cole song "Mona Lisa."
This concert poster for Frank Zappa and the Mothers of Invention. Referred to as the Mona Zappa.

Other

In Spaceship Earth at Epcot, one scene is the lady posing for the painting and rapidly losing her patience because Leonardo keeps on getting distracted by other projects.
In the card game Chrononauts, there are a set of three Artifact cards: Mona Lisa (Obvious Forgery), Mona Lisa (Excellent Forgery) and Mona Lisa (Real Thing). When more than one is in play, the Excellent Forgery trumps the Obvious Forgery, while the Real Thing beats both.
The Mona Gorilla from National Lampoon Magazine.
The Mona Lisa is moderately common in photoshop contests featuring famous artworks. The Mona Leia is one of the better results of such.

Video Games

Animal Crossing: Wild World lets you buy the Mona Lisa from Tom Nook. He calls it the "famous painting".
There's also a mission in Elite Beat Agents where the agents help "Leo" paint his masterpiece. Try and guess what it is.
The iconic portrait of the Holy Mother Sophia in Xenogears.
One episode of the weird Cho Aniki shooter series has a floating Mona Lisa that shoots Eye Beams. No, really.

Web Comics

Rusty and Co has a painting whose eyes followed you (for a good reason). It managed to confuse umber hulks. See also the next page.
Schlock Mercenary had one alien guard who did Freak-Out seeing "undigested eyeballs in a pile of poo" look like they follow him.

Western Animation

Seen in the art collection of Atlantis in one SpongeBob SquarePants movie, with a squid.
Daria has a Mona Lisa smile, a description popular in Fanfic.
In Looney Tunes Back in Action, a pair of x-ray glasses is used on the Mona Lisa, revealing a bra underneath.
- In a Pepe Le Pew cartoon, "Louvre Come Back To Me", Pepe pursues his hapless female victim through the Louvre, finally chasing her through the museum air-vents. His distinctive aroma drifts through the museum, humorously damaging assorted masterpieces. The last shot is the Mona Lisa, speaking to the audience; "I can tell you chaps one thing, it's not always easy keeping this smile."
Courage the Cowardly Dog - the family visits the Louvre, and Muriel ends up being trapped in the Mona Lisa when a series of events causes all of the works of art to come alive.
In her first appearance (of two) on Darkwing Duck, Splatter Phoenix steals the Mona Lisa's smile with intent to ransom it back.
In Time Squad, It was this smile that convinced Leonardo to return from Dadaism to his classic style. He did ask the model to do a wider smile, but wasn't too pleased with her teeth.
All the works of art stolen by The Collector in the Sushi Pack episode "But is it Art?" were parodies of famous paintings, including "The Groaning Pizza," a pizza-faced person in Mona Lisa's pose, but with a distinct frown.

Real Life

Salvador Dali painted one with himself as the "lady".
- There are speculations that the Mona Lisa is Leonardo da Vinci's self-portrait.
Street artist Banksy has a work with Mona Lisa holding a rocket-propelled grenade on her shoulder.
Marcel Duchamp's "L.H.O.O.Q" is Mona Lisa, with a goattee and a mustache drawn on, and the letters LHOOQ (a french slang accronym meaning roughly "she has a hot ass")
- Further parodied with THIS Harry Potter fan-art.