A few sites to get you started.

Physical Science Specific Sites
Rader's Chem4kids
Rader's Physics4kids
The Science Desk
The Physics Classroom Tutorial

Earth Science Specific Sites
Physical Geography.net
Rader's Geography4kids (Earth Science)
The Planet Earth

Not Physical Science, but the yuckiest site on the internet! Explore and have fun!
Did you know Bill Nye has his own website?  Visit it by clicking here: http://www.billnye.com/ and learn how you can explore science at home!
The Exploratorium has cool information on the science of sports. the science of cooking, and many other topics!
More science fun on the internet:
Mad Science
Amusement Park Physics
Adventures with Science Bob

Do you need help writing a bibliography for your science fair research (or any other research)?
Visit EasyBib to help you along (use the MLA format).