There are lots of ideas for a science fair project on this website. http://www.usc.edu/CSSF/
This website gives a collection of links to pages with ideas for science fair projects including some in math. http://www.ipl.org/youth/projectguide/#ideas
Here's a comprehensive list of every science fair accessible through the Web. If you have time to dig, this site is perfect for you! http://physics.usc.edu/%7Egould/ScienceFairs/
This PBS science show lists how to do experiments. You may get some good ideas for a project for 4th and 6th grade. http://pbskids.org/dragonflytv/
Chemistry science fair topics! Fun! http://chemistry.about.com/popular.htm
This site has great ideas for the science fair. Be sure to scroll down after selecting science fair project ideas. The topics are listed below the registration line. http://www.sciencebuddies.org/
Bibliography directions are found on this website. http://www.hobbyhorsebooks.com/biblio.html
Science Fair Ideas for Grades 4, 6, and 8 http://www.terimore.com/default.htm