exploreMarsnow.org – A mars simulator presenting “plausible concepts for near future Mars missions drawing upon the depth of knowledge from experts around the world.” Very interactive.
Science
YouTube – Magic School Bus. My kids love these – they’re fun to watch and full of great info.
Sodaplay – models which can be manipulated.
Hans Bethe: Quantum Theory Made Relatively Simple. A video lecture.
Creation Kids – Positive character trait (& animal) lesson plans. Find Bible verses, enrichment activities, games, books for topics like Kind Kangaroos and Resourceful Rhinos.
Handbook of Nature Study. This is a blog written by a homeschooling mom who uses the classic book Handbook of Nature Study by Anna Comstock published in the early 1900’s. (You can read the entire text online here.) She issues weekly challenges (the blog author, not Anna Comstock) encouraging you to get out and nature walk. Here’s here explanation of how to use the book.
Neave Planetarium …the sky in your web browser. An interactive tool to study the sky.
This flower craft is a simple, fun activity anyone can do to learn how “capillary action” works.
Singing Science Records Fun songs to learn science!
Tom Lehrer’s “The Elements”. If you have reason to learn all the element, this would make it a lot easier!
Simple Machines Activities Nice site!
An Introduction to Black Holes From the site: “An easy to understand explanation of what black holes are.” Uh, I should probably say “easy to understand” is relative, but I’ll just trust that for anyone with a keen interest in astronomy and black holes, it’s easy to understand and therefore a good resource. 
The Why Files | The Science Behind the News “The mission of The Why Files is to explore the science, math and technology behind the news of the day, and to present those topics in a clear, accessible and accurate manner…We hope this information will help explain the relationship between science and daily life.”
Your Age On Other Worlds – Enter your birthday, hit calculate and see how old you’d be on other planets. Find out why the numbers vary so much! See also Your Weight On Other Worlds.
Build a Solar System – Fill in the size of your sun and then calculate the sizes of the other planets, the distance between them, etc.
We made a scale model of the solar system once on a soccer field. We used a soccer ball as our sun and were amazed when we ran out of room on the field for all the planets. It provides a great picture of the vastness of our solar system.
The Exploratorium’s Observatory – “Observe” from the comfort of your own home.
NSSDC Photo Gallery – These are amazing photos of the planets, comets, asteroids and more. Wow!
The Adventures of Herman the Worm – This is a very nicely presented study on worms which includes all kinds of facts and information as well as directions on how to make a home for them and keep them yourself. Great for elementary kids!
Hunkin’s Experiments – These are my kind of science experiments–short, sweet and to the point in a very un-scientific format.
Science Experiments at Steve Spangler Science – Many of these experiments have video clips of Steve Spangler on the news doing his experiments in the studio. The videos are fun to watch and fun to do. You must watch this video of the Diet Coke and Mento Super Soda Dispenser 3000 (click on the “video” tab about half way down the page). What a blast!
Can you read this? – A fun and interesting exercise demonstrating how amazing our brains are.
Or try reading your own paragraph all jumbled up.
Instant Anatomy – Taught by a retired surgeon turned professor, this site is well-organized. For the upper high school student.
Paper airplane – The best paper airplane in the world! – I’m not sure if it’s truly the best, but it is pretty darn good.
ZOOMsci – experiments for:
Nova’s Tales from the Hive
Eternal Sunset – see photos from places all around the world where the sun is setting
The Yuckiest Site on the Internet – covers bodily functions, animals and more. Great for the elementary school-age boy!
The night sky – an easy way to learn how to spot and identify constellations
eNature – a packed resource