USDA Hardiness Zones
2006 National Arbor Day Foundation Hardiness Zone Map
Probably the most useful site for locating your Hardiness Zone is located here. Just enter your zip code and your Hardiness Zone will be shown.
The benefit of Hardiness Zones is that it provides a starting point for planning which plants can winter-over where you live. However, there are a few drawbacks to the Hardiness Zone Map. It does not consider day length (changes considerably the further from the equator you go), snow cover (moderates soil freezing and insulates roots), humidity, frost, or soil moisture. Probably the biggest drawback is that it does not consider how warm your summer will be. The classic example is comparing the Shetland Islands north of Scotland and southern Alabama. Both are listed as bewteen Hardiness Zone 8-9. However, the Shetland Islands are sub-artic and southern Alabama is sub-tropical. There are almost no plants that can grow in both places.
AHS Plant Heat Zone Map
Sunset Climate Map
Here is a searchable map to find your Sunset Climate Zone, but I prefer going to the Sunset website and selecting the U.S. region here, as it provides a map as well as information on that growing region. Sunset also provides a pretty substantial searchable plant database for their Climate Zones on this page.
Combining Hardiness Zones, AHS Plant Heat Zones, and Sunset Zones, you will be able to more confidently chose plants that are well suited to your local conditions. These are great tools for planning your Permaculture System.
Here is a link to my article on Hardiness Zones for the World.
For my reference, I also want to add a Pacific Northwest Hardiness Zone Map as well. It is based on the 1990 data, but is still useful.