Reeko's Mad Scientist Lab has some cool experiments for kids. Living Things has lots of good general biology info and links to tons of related sites. The Microbe Zoo. Lots of interesting information about microorganisms. The Great Plant Escape.
Kids can learn about plants by solving a series of mystery games.
Newton's Apple has lessons and experiments that can be used in conjunction with or independent of the award-winning TV show. The Tech is an online interactive museum of technology. Kids can find out what they would weigh on another planet and more. Could be handy for chemistry students. Informative and cool nature site.