for anyone who lives in the general Boston city limits (including cambridge, somerville, brookline, jp, etc etc) I highly recommend checking out the Boston Public Library (right under the Hancock Tower). It's quiet, there's wifi, the Sebastian's cafe there has great tables and comfy chairs, and in the summer the courtyard in the middle is absolutely fantastic.

I usually work out of my condo's 2nd bedroom (what we call our office) but when I have the itch to get out and really get work done I head to the BPL.