Almost all of William Shakespeare's plays are based on pre-existing works, legends, and historical figures. Out of all his plays, only The Tempest seems to be an original plot by Shakespeare.
Romeo and Juliet was an adaptation of an Italian narrative poem called The Tragical History of Romeus and Juliet, which had been translated into English about thirty years before Shakespeare's play. The poem's basic story is itself very similar to Ovid's "Pyramus and Thisbe" in The Metamorphoses from Classical Mythology.
Titus Andronicus bears many similarities to another myth Ovid recorded: the story of Philomena. Lavinia actually grabs a copy of The Metamorphoses to tell her family what had happened to her.
Othello was taken from Italian author Cinthio's short story "Un Capitano Moro", in his anthology Gli Hecatommithi. In the original story, the Moor gets away with his crime for a while, Iago's motive is lust over Desdemona, and the moral is that European women should not fall in love with foreigners.
Macbeth was taken from stories Shakespeare found in Raphael Holinshed's Chronicles of England, Scotland and Ireland.
Hamlet was based on a legendary Danish prince of the same name. Scholars also suspect that Shakespeare's play was an adaptation of a previous play based the legend. No copies of the "Ur-Hamlet," if it ever existed, have survived.
The Tempest was inspired by an actual shipwreck caused by a hurricane; the description of Bermuda, where the victims landed, just sounded that cool to English audiences.
While Shakespeare's plots might not be as original as people think, his use of language certainly was, and that leads to this trope in and of itself. As quoted in The Story of English:
If you cannot understand my argument, and declare "It's Greek to me", you are quoting Shakespeare; if you claim to be more sinned against than sinning, you are quoting Shakespeare; if you recall your salad days, you are quoting Shakespeare; if you act more in sorrow than in anger, if your wish is father to the thought, if your lost property has vanished into thin air, you are quoting Shakespeare; if you have ever refused to budge an inch or suffered from green-eyed jealousy, if you have played fast and loose, if you have been tongue-tied, a tower of strength, hoodwinked or in a pickle, if you have knitted your brows, made a virtue of necessity, insisted on fair play, slept not one wink, stood on ceremony, danced attendance (on your lord and master), laughed yourself into stitches, had short shrift, cold comfort or too much of a good thing, if you have seen better days or lived in a fool's paradise - why, be that as it may, the more fool you, for it is a foregone conclusion that you are (as good luck would have it) quoting Shakespeare; if you think it is early days and clear out bag and baggage, if you think it is high time and that that is the long and short of it, if you believe that the game is up and that truth will out even if it involves your own flesh and blood, if you lie low till the crack of doom because you suspect foul play, if you have your teeth set on edge (at one fell swoop) without rhyme or reason, then - to give the devil his due - if the truth were known (for surely you have a tongue in your head) you are quoting Shakespeare; even if you bid me good riddance and send me packing, if you wish I were dead as a door-nail, if you think I am an eyesore, a laughing stock, the devil incarnate, a stony-hearted villain, bloody-minded or a blinking idiot, then - by Jove! O Lord! Tut, tut! for goodness' sake! what the dickens! but me no buts - it is all one to me, for you are quoting Shakespeare. (The Story of English, 145)
Unlike the stereotypical musical comedy, Oklahoma!! doesn't use the standard Opening Chorus; its opening number is a solo. But neither did half of the musical comedies that came before it; in fact, many of them didn't have an opening number of any sort, unless you count the short passage of nondescript music the orchestra plays while the curtain opens on a scene of expository dialogue. (And though Oklahoma! was indeed the first Rodgers and Hammerstein musical, they had first collaborated in 1919, when their careers had barely started.)
Though Broadway musicals almost never used screenplays as source material before The Fifties, there was a musical in 1919 based on a silent movie.
A Broadway musical about cats, based on a series of poems, and with considerably more dancing than plot? Shinbone Alley, which opened 25 years before Cats. However, Shinbone Alley failed to put its cats in any kind of cat costumes, which may help account for its far shorter run.
An unusual choice of source material for a musical, stuck in previews, ends up abandoned by its original writer due to horrible reception. Bono and The Edge wrote the songs. The show in question? A Clockwork Orange: The Musical, 20 years before Spider-Man: Turn Off The Dark.
It's no secret that the songs in Mamma Mia! are just recycled ABBA hits, but the plot isn't exactly original, either: the 1968 film Buona Sera, Mrs. Campbell used largely the same plot, as did Carmelina, a 1979 Broadway musical flop by Alan Jay Lerner and Burton Lane.
In Godspell, many of the lyrics that aren't from The Bible are from old hymns and prayerbooks.
One thing that practically everyone knows about 17th century English theatre is that women were banned from stage acting, forcing men to play female roles. While this is certainly not wrong, comparatively few people seem to know that the ban on female actors was lifted during the reign of Charles II, which began in 1660—less than half a century after William Shakespeare's death. Out of the 400-odd years that Shakespeare's plays have been performed, women have been allowed to play the female roles in them for roughly 350 years.