Hardiness Zone Charts
The most often used zone map is The United States National Arboretum USDA Plant Hardiness Zone Map
Click on any area within the United States to go to a close-up of that region.
Another interesting site is the National Gardening Association USDA Hardiness Zone Finder
These are only two of many sources on the internet. Just enter "hardiness zone map" in Google, and find lots more!