The USDA, Agricultural Research Service has released an updated plant hardiness zone map. The 2012 Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones.
The site is specifically designed to be Internet-friendly and incorporates a "find your zone by ZIP code" function, leaving nothing to interpretation. It also allows you to look at a detailed map of each state.