A Walk in the Woods...interactive actual walk through the woods
Leafsnap - an electronic field guide
Nature - BBC - amazing site with videos and information about nature!
Electric Circuits - a good interactive resource with information and activities about making electrical circuits
ElectroCityA superb site where you can build a city and make the citizens happy by keeping their environment clean. 
Learning Circuits - a good flash-based resource about electrical circuits. Users are show information and given tasks to complete
NewsELA - free current events website that has informative articles that can be adjusted by their lexile (reading) level with just one quick click.
Science on the SimpsonsBring the intellectual juggernaut, Homer Simpson, into your classroom with these Simpson science clips. 
Study Jams - ScienceA great site with activities, quizzes, videos and even karaoke about every area of the science curriculum. Very useful when introducing new topics
Bugscope Interactively control an $800,000 scanning electron microscope and use the build-in chat to send questions to scientists. Free.
NASA Photojournal - a good collection of images of the solar system and others space objects.
Smithsonian Education - Smithsonian in your Classroom
Build an AtomThis webpage allows students to view the structure of atoms from simple hydrogen and Helium all the way up to the complex Ununoctium.
BubbaBrain - A collection of multiple choice games on topics across the curriculum, including maths, science and English at different grade levels.
The Body ExplainedInformation on common simple body questions. Includes what causes your stomach to growl, why does your nose run when you cry, why your teeth chatter when you are cold, what causes blushing, and more.
Solar System - the planets by the National Aeronautics and Space Exploration.
Freezeray...This site offers a growing bank of imaginative, highly visual teaching-aids developed for use with interactive whiteboards. The resources are designed to be used as rich sources of visually stimulating material, making use of both animations and drag and drop interactivity.
Climate Change Videos...from Weather Underground