Kids Planet This site features fact sheets on over 50 endangered species, as well as games and activities from the Defenders of Wildlife.
Kratts' Creatures Information about lots of cool animals and some fun games to play brought to you by those crazy TV guys.
Mad Scientist Network This site has three divisions: "Ask a scientist" - includes archives of past questions (which can be sorted by grade level) and allows you to ask science questions; "MAD labs" - lots of experiments to do; and "Madsci library" - lists many other useful science websites.
NASA for Students Pictures of the Earth, planets, stars, and other cool stuff, as well as the stories and games related to those images.
OLogy Learn a lot about astronomy, paleontology, and other "ologies" at this site from the American Museum of Natural History.