Real-time online weather information site.  Click on the map or use the search option to access information on current weather conditions for the U.S. and the world.  The "Fast Forecast" portion provides access to historical weather data.

NOAA page devoted to U.S. climate.  Find data on precipitation or temperature for the U.S., including historical data from 1899 to present; climate extremes and weather events (e.g. hurricanes, El Niņo); and worldwide weather and climate event coverage.