But you may not have ever heard -- or at least ever seen it performed the original way -- by the stars of the original Broadway production. (Yes, "stars," plural. It's a solo number first, but at the end of the show, the two principals sing it as a duet.) It's not just the song alone, but they re-create the scenes with dialogue, as well.
This comes from a special on PBS, The Best of Broadway: A Celebration of American Musical Comedy. It's the original Desiree, Glynis Johns, on stage with Len Cariou who played opposite her as Frederik, re-creating their famous number. After having been in love many years earlier, when she was a young actress and he an up-and-coming lawyer, he's telling her that he has to refuse her new advances, since he's in love now with his much-younger wife. And then they append to that the finale, as the story's many complicated plots twist and turn to this conclusion.
By the way, in case you were unaware, Glynis Johns had earlier played the mother in the film, Mary Poppins, singing the song "Sister Sufragette." (And as timing would have it, I saw her over the holiday weekend when TCM ran my favorite movie, Around the World in 80 Days. She's one of the many cameos. Her scene comes near-ish the end with Hermione Gingold, as two lower-class barflies making a bet as to whether Phileas Fogg will make it around the world in time. And in a whimsical bit of history, the two actresses would team up 17 years later on Broadway appearing together in 1973 in...well, you have the answer right here at your fingertips...A Little Night Music. As if that wasn't enough for whimsy, Ms. Gingold played the Grandmother, mother to Ms. Johns' Desiree!!)
And for those keeping a scorecard, Len Cariou later appeared in another Stephen Sondheim musical, starring as a far-less genteel character, playing the title role of the original Broadway production of Sweney Todd.