Science & Math

How Stuff Works
http://www.howstuffworks.com/
Learn how everything works -- from SPF in sunscreen, to Patriot missiles!

Astronomy

NASA/National Museum
http://www.amnh.org/rose/

Biology

Endangered Species
http://eelink.net/EndSpp/

Kimball's Biology Pages
http://www.ultranet.com/~jkimball/BiologyPages/

Plants Database
http://plants.usda.gov/

Chemisty & Physics

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

Periodic Table of the Elements
http://www.chemicool.com/

The Laws Of Physics
http://www.alcyone.com/max/physics/laws/

Earth Science

National Geographic
http://www.nationalgeographic.com

National Oceanic and Atmospheric Administration
http://www.noaa.gov

Savage Earth
http://www.pbs.org/wnet/savageearth/

Savage Seas
http://www.thirteen.org/savageseas/

The Earth Fact Sheet
http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html

Math Sites

Convert it!
http://www.sciencemadesimple.com/conversions.html

How Many? A Dictionary Of Units Of Measurement
http://www.unc.edu/~rowlett/units/

Internet Mathematics Library
http://forum.swarthmore.edu/library/

Webmath
http://www.webmath.com

Science Projects

IPL Science Fair Project Resource Guide
http://www.ipl.org/youth/projectguide/

Science Fair Central
http://school.discovery.com/sciencefaircentral/