The Nocturnal is a "performance space," located at 1800 E Burnside, at 18th and Burnside in Portland, Oregon. Just recently renovated, it is easily recognized by its distinct, bright blue exterior. Located underneath the building is a restaurant/bar that is only open to those 21 and above, while the main, street level floor is a dance hall/concert room/art gallery. The Nocturnal's website advertises being a "...comfortable atmosphere for art and people..." that offers a place "...to give local talent a venue to present their work." In addition to the standard concert venue, The Nocturnal offers tango, salsa, and swing lessons every Sunday night, as well as opportunities for independent film and video screenings, and craft night.
I recently (January 23, 2004) stopped by The Nocturnal to watch The Decemberists and Corrina Repp, and was pleased not only by the performance, but by the venue. Now, I haven't been to many concerts (this was my first not in an auditorium), but I thought that it was very clean, and well designed, acoustically speaking.
Sources:
http://www.nocturnalpdx.com/frame_index.htm