This spider is not small, but really fabulously well camouflaged. I don't have an identification yet. Cambridge MA (Amesbury Street), 8/25/2014.
I was actually on an urgent errand when I spotted it; five photos later, I got a phone call suggesting that the problem had spontaneously resolved itself.
The interest in moths was in large part launched by acquiring an excellent field guide (the Beadle & Leckie Peterson Guide); I do not have an equivalent for spiders, though I have just ordered one. Suggestions welcome, either on the ID, or on a guide I might like.