My Top Links for General Science Images,Videos and Information:

ABC Science has an awesome collection of audio, video, images, games, experiments and more, including Dr. Karl, the Surfing Scientist, Sleek Geeks, Catalyst and Walking with Dinosaurs. They also have two email discussion lists (Science Matters and Teaching Science) that you can join to exchange messages with other users interested in ABC Science.
ARKive , endorsed by David Attenborough, this site has images and videos of life on earth, especially threatened species, as well as information for projects.
BBC Bitesize has a range of education materials including animations, games and quizzes. I have used these activities for Maths and science by embedding the links in my blog. They also have some interactive games that can be embedded.
CSIRO Education Excellent information and experiments, Science-by-email and Scientists in Schools program. Subscribe to Science By Email to get weekly e-newsletters with articles, activities and a quick quiz. There is also a Maths By Email e-newsletter.
HowStuffWorks includes the best videos about earth and life science, military technology, space and more. It has extensive information about how everything works, with clips sourced from the Discovery Channel and Mythbusters, among others.
SciencePhotoLibrary An amazing collection of quality science images. These are Copyright images for use in class only - cannot be uploaded on blogs.
Science Spot: Lots to explore here, with puzzles, ideas, trivia and a reference desk. Written by a very experienced science teacher, this site has links to lesson plans and teaching resources.
Science Class: is a similar site with hundreds of lesson plans, worksheets, links to resources and interactives from another very experienced science teacher.

Three links for visualizing the size of the universe down to the size of the atom:

Childrens Misconceptions in Science A fabulous list of common misconceptions that children bring to science classes in all areas.
Middle School Science: Biology, Chemistry, Physics and Earth Science lessons for the middle school.
Science Daily Keep up to date with current science news and articles about research around the world.
Science Kids: Lesson plans, simple experiments and easy to understand information for junior and middle years students from New Zealand.
Forensic Testing - The Carlson Company is a U.S. based DNA Testing and Expert Witness facility used by individuals as well as law enforcement.