Modern Formations is an art gallery and performance space located in the heart of Garfield. Although it doesn't host as many shows as it used to, I've found that its concerts are among the most interesting in the city, typically attracting bands that have a strong avant-garde or performative dimension. The space itself is very intimate--it can maybe hold 100 people or so--and it provides a great home for the local arts scene.
If I still lived in the city, I would make a special point of stopping here for any gallery crawl, reading, or special event.