The Music Meister

I'm the heart of swing, I'm the twist and shout.
When you gotta sing, when you gotta let it out.

—Sweet, Buffy the Vampire Slayer, "Once More With Feeling", "What You Feel"

The Music Meister is a character who causes others to break out into spontaneous song and dance. Often used as a justification for a Musical Episode, in which case Genre Savvy characters will question (possibly in song) why they are suddenly acting like characters in a musical.

To qualify as The Music Meister, a character must have an explicit power to cause others to break out in song, and the musical interludes should be commented on as being out of the ordinary.

Related to Crowd Song and Spontaneous Choreography, since that's what this person causes.

Examples of The Music Meister include:

Anime and Manga

The Pokémon mini-movie "Gotta Dance!" revolved around Meowth's newest invention, "The Dancin' Pokébaton", which causes all Pokémon who listen to it to uncontrollably dance.

Comic Books

The Muse from the Doctor Who comic strip "Planet Bollywood" in Doctor Who Magazine. The Muse was a robot created by the ruler of a planet to induce courtiers to perform musical routines against their will. An insurgent faction realised her powers could be used for evil. Smuggled off-planet, she was damaged in a spaceship crash and started inducing random musical outbursts in the local inhabitants.
The Maestro, a minor foe of the Justice League of America, could cause his victims to dance uncontrollably.
In one Nodwick story, Yeagar becomes this accidentally, after he asks Artax to cast a spell on him, to give him more impressive speaking skills. Of course, Artax never really read the full description of what exactly the spell does... so of course, everyone (heroes and villains alike) start randomly singing together in the middle of the fights. The expressions during some of the songs are truly priceless.

Film

Giselle from Enchanted, who comes from a universe where random musical numbers are common. Only a few characters seem to notice that the singing and dancing is out of the ordinary.
In Shrek Forever After, the Pied Piper is a bounty hunter who uses his flute to capture his quarry by forcing them to dance.
On The Mask, the title character is surrounded by police, so he starts singing a mambo number and the policemen start singing and dancing along, to their great surprise.

Literature

Not a person, but otherwise along the same lines, is Rojahama's Song-and-Dance from the Doctor Who New Adventures novel Sky Pirates!. It's a force of nature, or perhaps some kind of meteorological effect, that causes spontaneous outbreaks of Crowd Song. (The planet on which this occurs is in a solar system that, for reasons explained later in the book, is basically one giant Weirdness Magnet.)

Live-Action TV

The demon Sweet from the Buffy the Vampire Slayer episode "Once More With Feeling" Upon his arrival in Sunnydale, everyone in town begins breaking into musical numbers; even a few vampires and demons get into it while planning to sacrifice a man for a graveyard ritual. While this seems harmless enough, as Dawn notes, the songs are always brutally honest ones that reveal people's deepest secrets, which sows discord among the populace. They also cause certain victims to spontaneously combust once they've revealed all their secrets or succumb to despair. Sweet realizes that, since he knows that Buffy is in a funk after her friends resurrected her, he can make her burn and uses Dawn as bait. So he invites her to try and defeat him; she starts to sing and dance instead about life. Her friends attempt to help, only for Buffy or reveal in front of her friends that they pulled her from heaven, not hell, and thus she's in a constant hell. They stand frozen as she starts to dance and smoke; Spike has to intervene and tell Buffy that life isn't a song or happiness, calming her down.
Xena's son Solan in Xena: Warrior Princess qualifies as this, as he is the driving force behind Xena and Gabrielle's being transported to the musical world of Illusia after they tried to kill each other.
- Also from Xena, Terpsichore's Lyre, from "Lyre, Lyre Hearts on Fire" is an inanimate version of this, as the musical aspects of the episode only begin when the Lyre is unearthed by Draco, and are abruptly ended when the Battle of the Bands is won. By Xena
The Imagin Ryutaros from Kamen Rider Den-O has the unique power to make everyone around him spontaneously burst into breakdancing with his theme song in the background just by snapping his fingers. After the first time, he even has a 'posse' that shows up whenever he wants someone to dance with.
Scrubs had this as a way to get in a musical episode. The source of the power? A patient's brain aneurysm.
In an issue of Doctor Who Magazine, Rory William's actor Arthur Darvil confessed to discussing the idea for this type of episode, with a monster/alien that makes everyone sing. Will this materialise? Time Lord knows.
In the third season of Community, Mr. Rad not only acts as a Music Meister, but slowly converts the study group into Mini Music Meisters.
The Luvvie tribe in The Legend of Dick and Dom episode "The Land of the Luvvies" has this effect on people.

Theatre

The Balladeer and, to a lesser extent, the Proprieter in Assassins have elements of this.
The Hatchetfield franchise has the Hive in The Guy That Didn't Like Musicals, an alien species that kills the humans before puppeteering them in musical numbers. They also have access to the individual's memories and personality, using them to lure in more victims. Even worse, Nightmare Time confirmed that the Hive, real name Otho, is the weakest of the family of Eldritch Abomination beings that come to plague Hatchetfield.

Video Games

Purge from the Space Channel 5 series. He can make people dance through his moves and hypnotism, but if that wasn't enough, he also has a Ballistic Groove Gun. Said gun can make an ENTIRE GALAXY dance when completely charged.

Western Animation

The Trope Namer is the Music Meister from the Batman: The Brave and the Bold episode "Mayhem of the Music Meister", a zoot suit-wearing hypnotist who can command people to do his bidding via song. He doesn´t have to sing his command, he just has to sing anything. Fortunately, his Evil Plan is to simply have his brainwashed backup singers and dancers deliver him all the wealth in the world. Unfortunately, he's willing to kill to make that plan happen, especially when Batman gets involved.
In My Gym Partner's a Monkey Presents Animal School Musical, the magical fish David Coppertrout grants Jake's wish to turn the world into a musical. The other characters are not impressed, even to the extent of singing songs about how terrible it is living in a musical.
In The Penguins of Madagascar episode "The Return of The Revenge of Dr. Blowhole", Dr. Blowhole's new device accidentally fuses with an experimental power cell and an MP3 player to form a machine capable of doing this. Blowhole himself becomes this when he takes control of it. Like the Trope Namer, he's voiced by Neil Patrick Harris.
My Little Pony: Friendship Is Magic: Pinkie Pie, so far, has started/led most on the songs in the show, and seems also capable of causing others around her to participate, as seen in "A Friend in Deed".