Ask an expert - http://www.askanexpert.com/ or

http://www.teach-nology.com/teachers/ask_experts/science/ or

http://www.allexperts.com/ or http://science.hq.nasa.gov/ or

http://www.vrd.org/locator/subject.shtml#science  Also see Mrs. Bisbe's Science Links.

Chemistry experiments - http://chemistry.about.com/cs/homeexperiments/

Electrifying adventures -http://www.hunkinsexperiments.com/themes/themes_electricity.htm

or http://www.exploratorium.edu/snacks/iconelectricity.html.edu/snacks/

Electron microscope images -www.pfizerfunzone.com/funzone/discovery/

Elements, some fun ways to learn about them -
www.chemsoc.org/  or http://chemlab.pc.maricopa.edu/periodic/periodic.html

Genetics info - http://www.genecrc.org/index.htm

Geology, earthquakes, and volcanoes -- www.usgs.gov/

Graphs, draw a graph - http://nces.ed.gov/nceskids/graphing/

How things work - www.howstuffworks.com

Human genome project - http://www.ornl.gov/hgmis

Maps -www.topozone.com or www.usgs.gov/

Nervous system activities -http://faculty.washington.edu/chudler/experi.html

Projects - www.eskimo.com/~billb/

Science news, science of sports, space science, science, science, science! - 
www.exploratorium.edu

Space stuff - www.nasa.gov/ or www.explorescience.com/