The Exploratorium provides hundreds of science activities based on their museum exhibits.  There are online activities, exhibits, webcasts, and detailed instructions for hands-on activities.

NG Kids has more than a dozen experiments and activities, tells you what you will need to do them, provides step-by-step directions and illustrations, and wraps up the “why?” of the science behind what makes it work.

From site:  If you like to perform science experiments, we have a ton of them that will keep you busy….

Funology.com is the best place on the Web to find things to do. Not just while you’re sitting at the computer, but anywhere. Things to do by yourself or with your friends, your parents, your pets, anyone. We’ll teach you a million and one ways to make things, explore the world, and discover skills you never knew you had. You’ll use words, numbers, facts, ideas, and objects you find around the house. It’s one place where you’ll never be bored!

From site: Welcome to the ever-growing, ever-changing collection of Steve Spangler’s favorite science experiments, recipes and cool science projects. This hands-on science library represents Steve’s most requested science experiments from his weekly television appearances and live presentations throughout the country. Just search for an easy science project idea or browse the entire library of experiments.

From site:  Welcome to Reeko’s Mad Scientist Lab. Your source of free science projects and experiments for parents, teachers, and children of all ages. Kick off your shoes. Pull up a chair. Make yourself comfortable. Oh yeah, did we mention that protective goggles are required…