National Park Services - biology, geology, ecology, history.
US Geological Survey Water Science - resources for activities related to water. [biology, geography, ecology]
The Smithsonian's "Earth From Space" explores the beauty and utility of satellite technology. Resources, lesson plans, ties to standards are provided.
VolcanoWorld organizes a great deal of information and web links about volcanos. Some activities are specifically designed for young learners. [geology]