The United States Department of Agriculture (USDA) has published a revised map dividing the country in to hardiness zones based on the average low temperatures one can expect. The look-up tool from the National Arbor Day Foundation below uses your ZIP code to determine your zone. The USDA hardiness zone map also has a zip code locater now and it seems to be a little more granular in its results, giving you a zone 5a or 5b instead of 5, for example.
It's been our experience that these two tools provide generally accurate, but broad descriptions of what your plants can expect to endure. Within these zones, however, exist micro-climates that can either add protection or expose you plants to killing temperatures. A good example of this is what you've probably experienced in your own yard. The North vs. Southern exposures as well as East vs. West all offer different growing conditions.
Here in Northeastern Ohio for example, areas along Lake Erie can stay colder in the spring and warmer in the fall due to the buffering effects of the lake. Our garden is approximately 20 miles south of the lake and early spring day time temperatures in those 20 miles can vary by 20 to 30 degrees, depending on wind direction, another factor that can influence temperatures greatly.