I love The Weather Channel website!
http://www.weather.com/
I always have some links to them in my signature block. The degree-day calculator is awesome. It uses observations for past date ranges, and predictions or long-term averages for future date ranges.
And all their features work "by ZIP code". I'm not sure if there is a way to tweak that for valleys and mountain slopes where one ZIP code spans very different micro-climates.
degree-day calculator
http://www.weather.com/outdoor...
30-day forecast plus last month's temps & precip:
http://www.weather.com/outlook...