GALILEO- Sponsored by the University System of Georgia. Results can be sorted by Lexile level and age.
Factmonster - Online reference books, homework help, games and more
KidzSearch - Family-friendly search engine powered by Google
Kidtopia - Another search engine by Google that only indexes educator approved web sites
National Geographic for Kids - A few links may require a subscription but plenty of free information about the natural world.
KidsClick! - Modified by librarians, can search by topic, reading level or picture content