Science for Kids - has science experiments, cool facts, online games, free activities, ideas, lesson plans, photos, quizzes, videos and science fair projects.
Sid the Science Kid - based off of the PBS cartoon series, this site offers videos, experiments and resources you can use.
Have Fun Teaching - offers science worksheets
Science with Me - a hands on educational science site.
The Naked Scientist - Kithcen science experiments
Science for Preschoolers - as the name says, it's a site dedicated to preschool level science.
Kids National Geographic site
Christian Answers - A christian science site for kids
Discovery Magazine - not affiliated with discover magazine or kids discover
Nature Detectives - ideas, printables, lessons, coloring pages, games, nature walk, crafts all related to nature, outdoor, seasons and the like
National Wildlife Federation for kids - the makers of Ranger Rick and Your Big BackYard Magazines, lots of nature and science activities, crafts and ideas