It’s the XML feed that Google uses to provide weather data. I’ve used Yahoo!’s weather feed before, but it’s not as complete. And The Weather Channel’s feed is more complex than I care to deal with. (It includes a developer kit!) Like Goldilocks said, Google’s seems just right. It includes the current conditions, four-day forecast, and even wind data.
The only downside is that you have to provide the city and state name. Supplying just a zip code would be much simpler. So, on a hunch a tried doing just that and it worked, too!
All that said, please remember that this feed is private to Google and not publicly endorsed (as far as I know). So use it sparingly if at all.