One of my (nursery) trade magazines had an article about the new Plant Hardiness Map, recently released by USDA.
A couple of neat features are; you can click on your state, and see all of the zones in your state, and you can type in your zip code, and find your specific zone.
USDA Plant Hardiness Zone Map
I'm not sure, though, how accurate this last is. For New York, they show the highest zone is 7b. I'm on the southern end of the south shore of the southern-most county in the state, but when I type in my zip, I get 7a. When I really enlarge the map, (to the point wher it gets a little blurry) the color shows we are 7b.
On the old map (from years ago) they said we were zone 6, with a first average frost date of Oct, 20. For the last several years, I got by until about election day. (varies from Nov.2 to 8) This year, we didn't get it until Dec. 12. It also states that, for zone 7a, the average low temperature is between +5 & +10F. Our low this year was, one night, at +13F.