Have some fun looking around these sites. You may find them quite helpful!
National Geographic for Kids http://www.nationalgeographic.com/kids/
Kidport Science Reference Library http://www.kidport.com/RefLib/Science/ScienceIndex.htm
Biology4Kids http://www.biology4kids.com
NASA for middle-school students http://www.nasa.gov/audience/forstudents/5-8/features/index.html
Chemistry for Kids http://www.chem4kids.com/
Physics For Kids http://www.physics4kids.com/
NOVA Interactive Archives http://www.pbs.org/wgbh/nova/hotscience/
Life Science Homework Helper http://www.homeworkspot.com/middle/science/lifescience.htm
Botany For Kids By Kids http://www.kidsturncentral.com/links/botanylinks.htm
Zoology Links http://www.kidsturncentral.com/links/zoologylinks.htm
Worls Biomes http://www.ucmp.berkeley.edu/exhibits/biomes/index.php