Most recently I used Parelli Optical in Porter Square (upstairs from Anna's), and it was fine.

You might consider the online-offshore options, which are crazy cheap; I recently got a pair of prescription sunglasses for $20 with shipping from Zenni Optical, and I have friends who have done well with Optical 4 Less. I'm not quite ready to trust my primary glasses to them, but it's cheap enough that I'm willing to go through a few secondary pairs.