Astronomy Picture of the Day - features a different image or photograph of our fascinating universe every day, along with a brief explanation written by a professional astronomer.
NOAA: Space Environment Center - provides space weather alerts and warnings to the nation and the world for disturbances that can affect people and equipment working in space and on Earth.
SOHO: The Solar and Heliospheric Observatory - NASA/ESA spacecraft designed to study the internal structure of the Sun, its extensive outer atmosphere, and the origin of the solar wind.
Solar Max 2000.com - offers a look at the solar maximum of the current solar cycle. With video clips, educational resources, and more. From the Exploratorium.