PBS' Nova Online deals with numerous science topics. The Discovery Channel's Web site has lots of interesting articles and activities for students and teachers. Dan's Wild Wild Weather Page is a great site for young students. It explains weather terms and concepts in easily understood terms. There are archives of questions, links and more.
Files has interesting news and archived articles about different issues and topics in science.
The always cool Bill Nye the Science Guy. Good general science resource site that has articles, experiments and more. Cool biology site that has information on cells, bacteria, viruses and more. Important health information for kids and parents. Science Education Gateway has lessons for different age groups covering areas of science such as weather, light, the solar system and more.