BBC Bitesize - Kindergarten-1st grade, 2nd grade to 5th grade, middle school, junior high science curricula.
Beacon Learning Center - I haven't figured out how to filter out only the science ones. On the subject line it will tell you if it is a science lesson and for what age group.
Discovery Kids - Kids can play games, watch movies, and explore different areas of science.
Easy Peasy - Full curricula for 1st grade - 8th grade on the science topics of biology, zoology, earth science, and physics/chemistry.
Easy Peasy for Highschool - Full curricula for high school on the science topics of oceanography, biology, physics, earth science, and chemistry.
Education World - A bunch of science lessons.