CAROLINE OR CHANGE is loosely based on Tony Kushner's childhood.
I don't think just because something is based on real events it doesn't qualify it as "original."
A CHORUS LINE is definitely an original piece regardless of the fact it is based on the tapes.
WILL ROGERS FOLLIES, TITANIC and 1776 are original as well.
I think sometimes we get the most creative musicals when they are adapted from something else, for example, GREY GARDENS.
Smaxie, you're right, I forgot CURTAINS (don't know how, it's one of my favorites playing right now).
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"