This site is a great way to create, share, and collaborate on physics lessons that you find on the site or design yourself. Watch this video to find more about Knotebooks.
PBS Teachers: Science & Tech
PBS includes free lessons, worksheets, and professional development opportunities in Science & Tech. Resources are organized by grade level, and they offer a PDF that provides an overview of the Science & Tech resources.
Bill Nye the Science Guy
Bill Nye has dedicated a section of his website to kids and teachers, where you can find videos, episode guides, printables, and kid-friendly science experiments called "Home Demos."
Google Earth Ocean
Google Earth 5.0 added the Ocean functionality. It allows you to explore oceanic elements, undersea wildlife, endangered species, and climate change with experts from the National Geographic and the BBC. Watch this video about Google Earth Ocean.
Tons of free resources for biology teachers. There are presentations that include topics organized by contents standard, teaching strategies, lab techniques, and more. You'll also find classroom lessons, units, slide sets, and a plethora of other classroom resources.
The US Geological survey has tons of free materials for primary, secondary, and undergraduate level students and teachers including online lectures, maps, lessons, activities, photos, videos, and more.
Interactive Periodic Table of Elements
At first glance, this looks like a basic periodic table. However, when you click on an element, you get a popup with information about that element including the atomic number, atomic symbol, atomic weight, family, etc. It also provides links to other online resources about that element.
Please comment and let me know what science resources you're using in your classroom!