I found this small-even-by-Tokyo-standards "piano bar" while lost in Nakano one night. The system is quite unique; they have live music a couple times a week but on "bar nights" customers are welcome to start jamming or rehearsing, using the grand piano that's wedged in the place or by bringing their own instruments. While sitting at the tiny bar in the corner one night I saw two guys walk in and order drinks, then start playing trumpet and bass. Very casual, very cool.

Pignose is a real neighborhood joint, very cramped but really friendly. Highly recommended.