I was comparing my energy usage from the last 2 years and was surprised to find that despite what I perceived to be a more mild summer we had more cooling degree days this year. Even the local weather forecasters have been noting our mild summer. This helps show the benefits of using actual weather data!
AC Hack posted a link here a while back for Heating Degree Days. Its the same site. It also has a few good articles on using the information.
Heating & Cooling Degree Days - Free Worldwide Data Calculation
Word of warning it uses a base temp of 65f, you can adjust it, I raised mine higher to an outdoor temp that would make it more likely to require air conditioning.