About.com Chemistry - a good general site covering a wide variety of chemistry topics
B J Pinchbeck's Homework Helper Science - covers a wide array of the sciences
General Chemistry Online - includes an index of self-guided tutorials, quizzes, and drills on specific topics
How Products Are Made - explains and details the manufacturing process of a wide variety of products, from daily household items to complicated electronic equipment and heavy machinery. Covers everything from bubble gum to telephones to washing machines
How Stuff Works - a fantastic site, covers from toasters, to Compact Discs, to phones, to cars et., etc., etc.
How Things Work - if it isn't in Stuff then it's probably here
Periodic Table Of The Elements - pick element, any element and get a synopsis of its uses, properties and history
Physics Classroom - an ideal starting location for those grasping for understanding or searching for answers
Science.ca - the very best place on the web for Canadian science
Smithsonian Education Science & Nature - explore, discover, learn science and nature
Toronto Public Library ScienceNet -links to biology, chemistry, physics, zoology, geology, astronomy, botany and paleontology. Lots of Canadian content