There are several out there. They were on the How far along are your trees? - #316 by tonyOmahaz5 thread
- weather.com version
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/cdus/degree_days/grodgree.txt
- NOAA version, quick but it seems to be a week out of date.
http://www.potashcorp-ekonomics.com/tools-to-calculate-fertilizer-needs/calculators/gdd/
- another one, I think corn is 50F so enter that.