Discovery Channel's Science Fair Central
http://school.discovery.com/sciencefaircentral/ 

General Chemistry Online!
http://antoine.frostburg.edu/chem/senese/101/index.shtml

HowStuffWorks: "Science Channel"
http://science.howstuffworks.com 

Learn Physics Today!  Online physics tutorial.
http://www.thinkquest.org/library/site_sum.html?tname=10796&url=10796/

MIT's Invention Dimension
http://web.mit.edu/invent/invent-main.html   Inventor profiles, handbook, games, links to other resources, and more.

Molecular Expressions
http://micro.magnet.fsu.edu/index.html   Explore optics and microscopy by visiting the photo galleries.

National Academy Press
http://www.nap.edu   The National Academies : "Advisers to the Nation on Science, Engineering, and Medicine." Choose a category, select from more than 3,000 books, and read them online FREE!

Periodic Table of the Elements
http://pearl1.lanl.gov/periodic/default.htm   Click an element for more information.

Public Library of Science
http://www.publiclibraryofscience.org

SciCentral
http://www.scicentral.com   Gateway to the best scientific research news sources.

Scirus
http://www.scirus.com/srsapp/   Science-specific Internet search engine.

Math and Science Gateway
http://www.tc.cornell.edu/CTC-Main/Services/Education/Gateways/Math_and_Science/ - Cornell Theory Center

Dictionary of Units of Measure
http://www.unc.edu/~rowlett/units/index.html   From the Metric System to hat sizes, and "everything in-between."

Illuminations: Selected Math Web Resources
http://illuminations.nctm.org/swr/index.asp - A joint venture between the National Council of Teachers of Mathematics and MarcoPolo.

The Math Forum
http://mathforum.org/dr.math/   Have a math question? Ask Dr. Math.

Mathworld
http://mathworld.wolfram.com "The web's most extensive mathematics resource."