Hi,
In Europe at least, http://www.awekas.at seems to be quite popular. Most of the weather stations have also solar radiation data. There is also information about the make and model of the particular weather station. The only problem is that there’s no API (or at least I couldn’t find one). Anyway, one of the graphs displays the selected weather data in a rolling 24 hour period, which is quite convenient for calculating and comparing the ET values.
Alex