This little event was kind of a ritual when I lived in NYC. So, if you are living in NYC or visiting on a Wednesday, you need to experience the bluegrass jam at the Grisly Pear. Basically, every Wednesday night around 9:30, you can grab a drink at this little bar in the village and listen to an acoustic jam of anywhere from 15 - 30 bluegrass musicians. They play all the bluegrass standards - some you probably know and some you don't.
Being from Texas, these Wednesday nights would transport me to some back room in Gruene, TX where I was sure the lightning bugs were twinkling outside along the river. Here's a little article the New York Times published in February on jam seshies in NYC, with a notable mention of the "Sherriff of Good Times."
I promise this will be one of your most memorable NYC experiences! Oh, and it's totally free, btw. Plus, once your done taking in all of the bluegrass, you can jump across the street to the Old Rabbit Club for a lovely import brew or wine.