this event last year, and they're doing it again - November 5, just in time for you-know-who's day.
What is it? Shakespeare, Open Mic. If you've got something to perform, put your name on the list and get up and do it!
I didn't love it last year, but that was mostly due to the circumstances. I'd brought the kids, but it's not really a kid-friendly place (practically standing room only, and getting food in the bar's backroom where it was held proved pretty tricky). Plus I had no idea what to expect, and what I saw looked to me like a bunch of people who already knew each other, getting up and doing set pieces. I wasn't sure where the open part was coming from.
But I spoke with the organizer after the event (that's what that second link is, up there), and he told me that while yes, they did deliberately schedule some "anchor acts" to make sure that the night had some structure, most of the acts were indeed just individuals who'd signed up and gotten up.
If you're in the neighborhood (Somerville, MA), go check it out! Don't bring the kids.