Depending on your definition of "underground"...here are some thoughts.
"Bare: A Pop Opera" is off-Broadway, but has a huge following of die-hard Barees.
"Brooklyn" is looking to be the next big thing to hit Broadway.
In performers, everyone has an underground following, defined as people not well known outside of Broadway, and even in Broadway not well known. Examples include:
Julia Murney
Jessica Grove
Michael Arden
John Hill
There are performers who, while loved and adored, have yet to get the "name above title" type of fame.
And that's all I can think of.
What college?
Deet: Shira, I Love You!