What word best describes a small store that sells only glasses (spectacles)?

How about "Optician's?"

I would not suggest the use of "glass" or "glasses"; it would not be specific enough to identify the store as somewhere eyeglasses are sold. There are stores that sell collector's edition glasses (as in those that you drink from), and so there could be some confusion involved.

I would also not suggest "optics" as there are other types of optics out there as well - lenses and such for cameras, for example.

"Opticians" (with or without the apostrophe) would imply that the store also provides eye exams, at least to me. If that is the case - that there is an optometrist on site - then this would be a good choice, regardless of whether it is in the UK or here in the US.

I think the K.I.S.S. approach would be best - "eyeglass store". This it to the point and does not leave room for doubt as to what is sold there. It isn't fancy or elegant, but it is about as accurate as you can get.

In the U.S., you should call it an "optical shop" (1.27 million google hits), which I believe is the most common term in American English for it. "Optical store" only gets 1/5 of the hits, although as nohat points out, this is what both LensCrafters and J.C. Penney call their optical shops. I think they're trying to make their locations sound a little fancier.

In Britain, as many people have pointed out, it's an "optician's".

For what it's worth, both LensCrafters and JC Penney Optical call their retail locations "optical stores".