Some descriptions below taken from lii.org.
Frank Potter's Science Gems
"Great links to Great Science Resources. More than 14,000 Science Resources sorted by Category, Subcategory, and Grade Level."
This Web site unites "hundreds of scientists in a forum where people can ask questions and learn more about the world around them." Questions submitted must be science related and are usually answered within seven days. Questions and answers are archived and can be searched by subject, keyword, and grade level. An excellent source for science project ideas, classroom activities, and trivia.
National Science Digital Library (NSDL)
"NSDL provides educational resources for science, technology, engineering and mathematics education." The site features links to hundreds of digital resources for teaching and learning about science. Material is searchable or browsable by topic. Also includes news and highlights from the collection. Searchable. Funded by the National Science Foundation (NSF).
A directory of links to "today's breaking science news." Browsable by topic, including biosciences, health sciences, physics, chemistry, earth and space, and engineering. Also includes links to related journals, databases, job opportunities, and conferences.
How Stuff Works
A commercial site that describes the mechanics of how things work. Subdivisions include explanations of automechanics, computers, electronics, and the human body.