A brief video profile of the Ebony Hillbillies
Jeremiah Lockwood performing in the New York subway.
The MUNY program includes performers in a wide array of styles. Aside from the conventional, you can see people playing saws, or Chinese dulcimer.
I should note that there are plenty of musicians who perform without the MUNY imprimatur -- that will be true as long as there is potential tip money. Quality can vary, of course, but some are really good. A number of CDs in my collection were unplanned purchases from musicians whom I liked.
Now sometimes the performers get on the trains to serenade the passengers between stops. There's one particular mariachi band that I've seen a bunch of times on the E and F trains, and around twenty or so years ago their was a group of breakdancers that always seemed to find my car on the 7 train. I'm more likely to be annoyed by performers on the trains than by those on the platform, only because the close quarters and closed doors on the train make the music more of an imposition on those who may not want to listen.
If you're into a variety of styles and want a free concert (though tipping, or buying a CD is always encouraged) during the weekday, you can always find something in the subway.