The Wood Street Galleries is a small gallery located in the heart of downtown above the T station. It has a layout similar to the Mattress Factory, but smaller (only two floors... usually with two to four installations per floor) and it usually, but not always has installation art. Sometimes the installations are really awe-inspiring (in my experience about 75% of the time); sometimes not so much. Admission is always FREE!
Details: To get inside, you need to enter through the door at Sixth and Wood St. (pretty sure about this...). From there, take the elevator (only one and kind of small) up to the 2nd or 3rd floor, where the actual galleries are.
Some advice: Try not to go during one of the quarterly Gallery Crawls because there'll be a looong line for the elevator to get in.