Photographic Periodic Table:
A complete periodic table reference containing just about all the information a student would need to know about each of the chemical elements. 

Periodic Table:
An online periodic table  of the elements brought to you by the Royal Society of Chemistry.

Science Homework Help:
Science resources, videos, and homework help from Discovery Education

Fact Monster: 
Provides essential reference materials, fun facts and features, and homework help.

How Stuff Works:
A source of unbiased, reliable, easy-to-understand answers and explanations of how the world actually works.

Unit Converter:
Converts angle, area, bits & bytes, density, electric current, energy, force, fuel consumption, length, mass, power, pressure, speed, temperature, time and volume units.Source:

Standard Atmosphere Calculator:
Computes atmospheric properties like density, temperature, pressure and speed of sound up to 86 kilometers altitude.Source:

NASA:
National Aeronautics and Space Administration website. 

EOL Encyclopedia of Life: 

Knowledge about life on Earth.