For me it was The Main Point in Bryn Mawr, Pa., just outside of Philly. I missed the first wave of the folk revival, but I remember seeing Tom Paxton, John Denver (1969), Steve Goodman (with the Manhattan Transfer as an opening act!), Bryan Bowers, Jonathan Edwards, Tom Rush, Hedge and Donna, James Taylor (1970), Jamie Brockett, John Renbourn...I wish I had saved all those schedules. I remember a powerful singer named Ellen McIlwaine doing a duet with the siren at the firehouse across the street. There's an old Oak publication called the Coffee House Songbook, and the photo on the front is of the interior of the Main Point. I wonder if it's still open.
I also was lucky enough to visit the Troubadour in London in 1978, where I heard Stefan Grossman give a little guitar lesson: "Andres Segovia doesn't play an F-chord this way, with his thumb...that's because he can't."