In Albert Herring, at the moment when Sid and Nancy pour rum into Albert's drink, an unremarkable ostinato is interrupted by the famous first chord of Tristan und Isolde, accompanying a viola solo very similar to the one Wagner wrote for when Brangäne offers the Love Potion to Isolde.
The title song of Allegro includes a short parody of "The Whiffenpoof Song":
They are smart little sheep who have lost their way. Blah! Blah! Blah!
Amaluna is loosely based on The Tempest as well as referencing other Shakespeare plays (e.g. the Romeo and Juliet-esque waterbowl falling in love scene), and the white-dressed Peacock Goddess and her black alter ego echo the White and Black Swans from Swan Lake.
The original London production of Chess had Anatoly watching television in a scene and at one point ABBA's "Money, Money, Money" can be heard, a shout out to the show's Pop Star Composers, Benny Andersson and Björn Ulvaeus of ABBA.
The 2010 UK Tour of Chess inserted a line from "Money, Money, Money" into the song "Merchandisers".
Midway through Coming Out of Their Shells, the turtles do a parody of Jeopardy!, with the question being "A turtle's favorite word would be this." A flawed parody, as they don't phrase the answer in the form of a question.
In Fiorello!, "Gentleman Jimmy" includes this nod to "Will You Love Me in December (as You Do in May)?", an old ballad whose co-author really was James J. Walker:
Say, Jim, we promise on voting day We will love you in November as we do in May
In Funny Girl, the World War I number "Rat-Tat-Tat-Tat" has Fanny, as "Private Schwartz", sing, "I met Mademoiselle from Armentières," referencing a rather notorious Bawdy Song popular among British soldiers of the period.
In "Rosemary" from How to Succeed in Business Without Really Trying, the lyric, "What a crescendo," is followed by an interpolation of the opening crescendo and a few bars of piano solo from Edvard Grieg's Concerto in A Minor. (The original orchestration requires one of the pit violinists to double as a pianist just to do this part.)
The opening/title number of In the Heights gives a shout out to Cole Porter, mentioning "Too Darn Hot" from Kiss Me, Kate.
In the Heights also contains shout outs to the song "Take the A Train", the Broadway star Chita Rivera, and It's a Wonderful Life.
In Carl Orff's opera Die Kluge, when the vagabonds talk of the power of luck, one of them says, "O Fortuna, velut luna!" This is, of course, the first line of Orff's setting of Carmina Burana.
Jury: Our object all sublime We shall achieve in time, To let the melody fit the rhyme, The melody fit the rhyme. Ringmaster: This is all immaterial and irrelevant! What do you think this is — Gilbert and Sellivant?
Tomorrow, you poor Jerseyites, who got such awful jars, When Orson Welles went on the air and made you all see stars, I know you'll be relieved to hear we're giving him back to Mars, 'Cause there ain't gonna be no sorrow, tomorrow.
In Let 'Em Eat Cake, Kruger, in his nihilistic List Song, says, "Down with pianists who play 'Nola'!" The first two bars of "Nola" are then interpolated.
A very subtle one from Gilbert and Sullivan's The Pirates of Penzance: "Poor Wandering One" is really, really similar to "Sempre libera" from Verdi's La traviata. Most likely intentionally, consideirng that "la traviata" means "she who has strayed," which is the subject of G&S's song....
Also done in The Pirates of Penzance, where Major General Stanley sings that he can "whistle all the airs from that infernal nonsense Pinafore", referring, of course, to Gilbert and Sullivan's previous hit, H.M.S. Pinafore. The finale originally included a direct paraphrase of H.M.S. Pinafore:
Girls: Oh, spare them! They are all noblemen who have gone wrong! Major-General: What, all noblemen? King: Yes, all noblemen! Major-General: What, all? King: Well, nearly all!
RENT gives a specific shout out to La Bohème, the opera it's based on, in "La Vie Boheme" when Mark remarks Roger's song sounds like "Musetta's Waltz". Roger's guitar motif is the opening phrase to "Musetta's Waltz." A more subtle reference to the same aria occurs in "Take Me or Leave Me," where Maureen's first verse (where she describes people admiring and flirting with her as she walks down the street) has the same basic theme as "Quando me n'vo." The opening number contains another riff taken directly from La Boheme.
Ironically enough, the same riff is used in Lloyd Webber's "Make Up My Heart" from Starlight Express.
Then there's "Christmas Bells", which in many ways is a Darker and Edgier riff on La Boheme's holiday street scene. Most notably, the children chasing after the toy seller Parpignol in the original become drug addicts trailing after "The Man", their supplier.
William Shakespeare's plays include several Shout Outs to earlier Shakespeare plays. Notably, Hamlet includes several references to Julius Caesar. At one point Polonius claims to have played Caesar on stage, almost certainly indicating that the actor who originally played Polonius had previously played Caesar in Shakespeare's version. (Like Caesar, Polonius is also stabbed to death, although in his case it's due to mistaken identity.)
In Puccini's one-act opera Il Tabarro, the refrain to the balladeer's song of Mimi includes an instrumental quote of "Mi chiamano Mimì" from La Bohème.
Tick Tick Boom!, another Jonathan Larson musical, is filled with shout outs to Stephen Sondheim. The biggest of these is the song "Sunday", which is an update of a song by the same name in Sunday in the Park With George.
Shakespeare's Troilus and Cressida quotes Marlowe's Doctor Faustus on Helen of Troy. This sounds slightly weird to a modern audience, as the line in question ("Was this the face that launched a thousand ships?") has become cliché — it's probably the most famous sentence Marlowe ever wrote.
The mirror scene in Shakespeare's Richard II could well also be a reference to that same line.
In the original Russian version of Uncle Vanya, towards the end of Act 3, the professor announces to his assembled family members that a government inspector is coming, before launching into his speech. It is a reference to an earlier play, The Inspector General, by Nikolai Gogol.
During the "Guardian Angel in the City" act, Waz envisions himself swinging from a chandelier. Ironically, Cirque du Soleil's subsequent ice arena show, Crystal, used a cover of "Chandelier" during a similar aerial number, albeit with a swing instead of a lamp.
The white-dressed woman's hairstyle and costume are reminiscent of Daenerys Targaryen from Game of Thrones.
The second verse of "To the Stars" paraphrases the beginning of "Defying Gravity" from Wicked: "Something has changed within me. Nothing will ever be the same. I'm done playing with logic. Of someone else's game."
In the aforementioned Cirque production Crystal, the projection of the eponymous heroine breaking through the ice to the surface at the story's climax is reminiscent of the video to Lindsey Stirling's "Shatter Me".