Astronomy
Galaxy Zoo: Help astronomers identify galaxies! Really!
Solar System Simulator: NASA's view of the near universe.
Biology
Classification of Living Things: Linnaean classification explained.
Genetics: Clear, engaging explanations.
Anatomy: Explore a human body map in 3-D.
Molecular Movies: Beautiful and informative simulated videos of the life of cells.
Chemistry
Periodic Table of Videos: Each element in video!
Mathematics
Mathworld: Serious mathematics website; "the web's most extensive mathematical resource, provided as a free service to the world's mathematics and internet communities.
Middle School Math: Pre-algebra and more, printable worksheets.
What's Special About This Number: Properties of numbers, examples.
Wolfram Alpha: computational knowledge engine.
Miscellaneous
Khan Academy: Non-profit educational website offering interactive lessons on math, science, computer programming, history, art history, economics, etc.
Cloud Appreciation Society: Photos, info.
Conversions: Convert just about anything to anything else;5,000+units,50,000 conversions.
Exploratorium: Articles & images about science, art and human perception.
National Ocean Service: Know your waters.
Office of Scientific & Technical Info, U.S. Department of Energy: a wealth of research material.
Science.gov: portal to the content of 13 federal agencies.
Technology
Technology Timeline
Timeline: Telephone oriented,1800's to 2005.