Earth Viewer is a slow but fascinating site. It allows you to create interactive views of the earth from any vantage point in space; here is your chance to watch the sun set from the space shuttle. The site also offers access to interactive planetary and sky maps.
WXP offers a comprehensive and intelligently packaged collection of current U.S. weather images, climatological data and forecasts.
Byrd Polar Research Center contains a diversity of information and images concerning all research polar or ice related; there is even a peek into the past, via the ancient climate records locked in ice cores.
The EROS Data Center, a part of the U.S. Geological Survey, is distributing declassified intelligence photos. This is the entry point for those who want to sample these spoils of the Cold War.
University Corporation for Atmospheric Research (UCAR) has created an abundant source for all kinds of weather information and predictions. UCARweb also contains background information on climate and environmental issues.
VolcanoWorld may look a little silly at first, but there is intelligence just below the surface. The site includes essays on volcanic processes and pointers to lots of images; just be prepared to dig around a little.